Make Clipboard argument in Widget trait mutable

graphics/src/widget/canvas.rs

@@ -154,9 +154,9 @@ where
         event: iced_native::Event,
         layout: Layout<'_>,
         cursor_position: Point,
-        messages: &mut Vec<Message>,
         _renderer: &Renderer<B>,
-        _clipboard: Option<&dyn Clipboard>,
+        _clipboard: &mut dyn Clipboard,
+        messages: &mut Vec<Message>,
     ) -> event::Status {
         let bounds = layout.bounds();