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/tour/src/main.rs

@@ -689,7 +689,7 @@ fn ferris<'a>(width: u16) -> Container<'a, StepMessage> {
     .center_x()
 }
 
-fn button<'a, Message>(
+fn button<'a, Message: Clone>(
     state: &'a mut button::State,
     label: &str,
 ) -> Button<'a, Message> {