Support conversion from Fn trait to custom theme

...instead of just from function pointers.

I'm making this change not because I actually want to pass a closure,
but to make passing a single fixed function work. This commit also
simplifies the scrollable example slightly, and without the other half
of this change that simplified example fails to compile with:

```
error[E0277]: the trait bound `iced::theme::ProgressBar: From<for<'a> fn(&'a Theme) -> iced::widget::progress_bar::Appearance {progress_bar_custom_style}>` is not satisfied
   --> examples/scrollable/src/main.rs:292:28
    |
292 |                     .style(progress_bar_custom_style)
    |                      ----- ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `From<for<'a> fn(&'a Theme) -> iced::widget::progress_bar::Appearance {progress_bar_custom_style}>` is not implemented for `iced::theme::ProgressBar`
    |                      |
    |                      required by a bound introduced by this call
    |
    = help: the trait `From<for<'a> fn(&'a Theme) -> iced::widget::progress_bar::Appearance>` is implemented for `iced::theme::ProgressBar`
    = note: required for `for<'a> fn(&'a Theme) -> iced::widget::progress_bar::Appearance {progress_bar_custom_style}` to implement `Into<iced::theme::ProgressBar>`
note: required by a bound in `iced::widget::ProgressBar::<Renderer>::style`
   --> /home/marienz/src/iced/widget/src/progress_bar.rs:77:21
    |
77  |         style: impl Into<<Renderer::Theme as StyleSheet>::Style>,
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `ProgressBar::<Renderer>::style`
```

This happens because `progress_bar_custom_style` by itself is a function
item, which is typically coerced to a function pointer when one is
needed, but not in this case. It is possible to work around this on the
caller's side, but especially since the compiler diagnostic for this is
a bit rough (see rust-lang/rust#100116) let's
try to make it work out of the box.

examples/scrollable/src/main.rs

@@ -289,18 +289,13 @@ impl Application for ScrollableDemo {
             }
             Direction::Horizontal => {
                 progress_bar(0.0..=1.0, self.current_scroll_offset.x)
-                    .style(theme::ProgressBar::Custom(Box::new(
-                        ProgressBarCustomStyle,
-                    )))
+                    .style(progress_bar_custom_style)
                     .into()
             }
             Direction::Multi => column![
                 progress_bar(0.0..=1.0, self.current_scroll_offset.y),
-                progress_bar(0.0..=1.0, self.current_scroll_offset.x).style(
-                    theme::ProgressBar::Custom(Box::new(
-                        ProgressBarCustomStyle,
-                    ))
-                )
+                progress_bar(0.0..=1.0, self.current_scroll_offset.x)
+                    .style(progress_bar_custom_style)
             ]
             .spacing(10)
             .into(),
@@ -372,16 +367,10 @@ impl scrollable::StyleSheet for ScrollbarCustomStyle {
     }
 }
 
-struct ProgressBarCustomStyle;
-
-impl progress_bar::StyleSheet for ProgressBarCustomStyle {
-    type Style = Theme;
-
-    fn appearance(&self, style: &Self::Style) -> progress_bar::Appearance {
-        progress_bar::Appearance {
-            background: style.extended_palette().background.strong.color.into(),
-            bar: Color::from_rgb8(250, 85, 134).into(),
-            border_radius: 0.0,
-        }
+fn progress_bar_custom_style(theme: &Theme) -> progress_bar::Appearance {
+    progress_bar::Appearance {
+        background: theme.extended_palette().background.strong.color.into(),
+        bar: Color::from_rgb8(250, 85, 134).into(),
+        border_radius: 0.0,
     }
 }

style/src/theme.rs

@@ -105,16 +105,18 @@ impl application::StyleSheet for Theme {
     }
 }
 
-impl application::StyleSheet for fn(&Theme) -> application::Appearance {
+impl<T: Fn(&Theme) -> application::Appearance> application::StyleSheet for T {
     type Style = Theme;
 
     fn appearance(&self, style: &Self::Style) -> application::Appearance {
         (self)(style)
     }
 }
 
-impl From<fn(&Theme) -> application::Appearance> for Application {
-    fn from(f: fn(&Theme) -> application::Appearance) -> Self {
+impl<T: Fn(&Theme) -> application::Appearance + 'static> From<T>
+    for Application
+{
+    fn from(f: T) -> Self {
         Self::Custom(Box::new(f))
     }
 }
@@ -363,8 +365,8 @@ pub enum Container {
     Custom(Box<dyn container::StyleSheet<Style = Theme>>),
 }
 
-impl From<fn(&Theme) -> container::Appearance> for Container {
-    fn from(f: fn(&Theme) -> container::Appearance) -> Self {
+impl<T: Fn(&Theme) -> container::Appearance + 'static> From<T> for Container {
+    fn from(f: T) -> Self {
         Self::Custom(Box::new(f))
     }
 }
@@ -391,7 +393,7 @@ impl container::StyleSheet for Theme {
     }
 }
 
-impl container::StyleSheet for fn(&Theme) -> container::Appearance {
+impl<T: Fn(&Theme) -> container::Appearance> container::StyleSheet for T {
     type Style = Theme;
 
     fn appearance(&self, style: &Self::Style) -> container::Appearance {
@@ -777,8 +779,10 @@ pub enum ProgressBar {
     Custom(Box<dyn progress_bar::StyleSheet<Style = Theme>>),
 }
 
-impl From<fn(&Theme) -> progress_bar::Appearance> for ProgressBar {
-    fn from(f: fn(&Theme) -> progress_bar::Appearance) -> Self {
+impl<T: Fn(&Theme) -> progress_bar::Appearance + 'static> From<T>
+    for ProgressBar
+{
+    fn from(f: T) -> Self {
         Self::Custom(Box::new(f))
     }
 }
@@ -808,7 +812,7 @@ impl progress_bar::StyleSheet for Theme {
     }
 }
 
-impl progress_bar::StyleSheet for fn(&Theme) -> progress_bar::Appearance {
+impl<T: Fn(&Theme) -> progress_bar::Appearance> progress_bar::StyleSheet for T {
     type Style = Theme;
 
     fn appearance(&self, style: &Self::Style) -> progress_bar::Appearance {
@@ -826,8 +830,8 @@ pub enum Rule {
     Custom(Box<dyn rule::StyleSheet<Style = Theme>>),
 }
 
-impl From<fn(&Theme) -> rule::Appearance> for Rule {
-    fn from(f: fn(&Theme) -> rule::Appearance) -> Self {
+impl<T: Fn(&Theme) -> rule::Appearance + 'static> From<T> for Rule {
+    fn from(f: T) -> Self {
         Self::Custom(Box::new(f))
     }
 }
@@ -850,7 +854,7 @@ impl rule::StyleSheet for Theme {
     }
 }
 
-impl rule::StyleSheet for fn(&Theme) -> rule::Appearance {
+impl<T: Fn(&Theme) -> rule::Appearance> rule::StyleSheet for T {
     type Style = Theme;
 
     fn appearance(&self, style: &Self::Style) -> rule::Appearance {