Do not require Copy for text StyleSheet::Style

For most widgets, `Style` only requires `Default`. A few require
`Clone`. Only this one requires `Copy`.

Some of the types in the default theme has a custom variant requiring
`Box<dyn Trait>`, or `Rc<dyn Trait>` to provide `Clone`, but this isn't
possible if `Copy` is required.

It would be good to also address the inconsistency of requiring `Clone`
in some places and not others.

This removes `style/src/text.rs` which is unused in this branch and thus
confusing. If there's a reason to keep it, that can be removed from the
change.

core/src/widget/text.rs

@@ -153,7 +153,7 @@ where
             &self.content,
             self.size,
             self.font,
-            theme.appearance(self.style),
+            theme.appearance(self.style.clone()),
             self.horizontal_alignment,
             self.vertical_alignment,
         );
@@ -233,7 +233,7 @@ where
             horizontal_alignment: self.horizontal_alignment,
             vertical_alignment: self.vertical_alignment,
             font: self.font,
-            style: self.style,
+            style: self.style.clone(),
         }
     }
 }
@@ -261,7 +261,7 @@ where
 /// The style sheet of some text.
 pub trait StyleSheet {
     /// The supported style of the [`StyleSheet`].
-    type Style: Default + Copy;
+    type Style: Default + Clone;
 
     /// Produces the [`Appearance`] of some text.
     fn appearance(&self, style: Self::Style) -> Appearance;

style/src/theme.rs

@@ -980,7 +980,7 @@ impl scrollable::StyleSheet for Theme {
 }
 
 /// The style of text.
-#[derive(Clone, Copy, Default)]
+#[derive(Clone, Default)]
 pub enum Text {
     /// The default style.
     #[default]