Require Clone for Message early when needed

Prior to this change, the widgets that needed a `Clone` bound on `Message` to
implement the `Widget` trait could be created with a non-cloneable `Message`.

As a consequence, the compiler complained only when actually trying to use the
`Widget` trait. Normally, this happens when trying to `push` the widget in a
container or turn it into an `Element`.

Furthermore, the compiler error in this case does not mention `Message` nor the
`Clone` bound, but instead complains about a missing `From` implementation.
Thus, it can easily cause confusion!

This change introduces `Clone` bounds in the main implementation of the
widgets that need it to properly implement the `Widget` trait. As a
result, the compiler complains early when trying to create one of these widgets
with a non-cloneable `Message` and explicitly mentions that the `Message` needs
to implement `Clone`.

examples/color_palette/src/main.rs

@@ -284,7 +284,7 @@ impl<C: 'static + ColorSpace + Copy> ColorPicker<C> {
         let [s1, s2, s3] = &mut self.sliders;
         let [cr1, cr2, cr3] = C::COMPONENT_RANGES;
 
-        fn slider<C>(
+        fn slider<C: Clone>(
             state: &mut slider::State,
             range: RangeInclusive<f64>,
             component: f32,