Built-in Theme customization

[hecrj]

This PR introduces a Custom variant to every style available in the built-in Theme. These new variants take a boxed StyleSheet of the widget with a Theme style. This allows for complete customization on a case-by-case basis of any widget without the need to implement a custom theme, which has quite the steep entry barrier.

For instance, the Button style is defined now as follows:

#[derive(Default)]
pub enum Button {
    #[default]
    Primary,
    Secondary,
    Positive,
    Destructive,
    Text,
    Custom(Box<dyn button::StyleSheet<Style = Theme>>),
}

Thus, any user using the built-in Theme can implement now a custom button::StyleSheet and have full control over the appearance of a Button. For example:

use iced::widget::button;
use iced::theme::{self, Theme};

pub struct MyButtonStyle;

impl button::StyleSheet for MyButtonStyle {
    type Style = Theme;

    fn active(&self, theme: &Theme) -> button::Appearance {
        // Complete control over the `button::Appearance` here!
        // ...
    }

    // ...
}

pub fn my_button_style() -> theme::Button {
    theme::Button::Custom(Box::new(MyButtonStyle))
}

And then you can use it on any Button:

button("I'm a button!").style(my_button_style())

Basically, this PR satisfies all of the use cases of our previous styling approach (before #1362 landed) while still encouraging a first-class theme with consistent styling.