Provide Renderer to Widget::on_event

This allows us to implement configurable event processing that adapts to
different rendering strategies.

native/src/widget/button.rs

@@ -31,6 +31,7 @@ where
         layout: Layout<'_>,
         cursor_position: Point,
         messages: &mut Vec<Message>,
+        _renderer: &Renderer,
     ) {
         match event {
             Event::Mouse(mouse::Event::Input {