Draft first-class Theme support

RFC: https://github.com/iced-rs/rfcs/pull/6

native/src/renderer.rs

@@ -9,6 +9,9 @@ use crate::{Background, Color, Element, Rectangle, Vector};
 
 /// A component that can be used by widgets to draw themselves on a screen.
 pub trait Renderer: Sized {
+    /// The supported theme of the [`Renderer`].
+    type Theme;
+
     /// Lays out the elements of a user interface.
     ///
     /// You should override this if you need to perform any operations before or