improv(menu): simplify menu construction.

- Added `MenuAction` trait to call the `message` method on button press.
- Added two new methods to construct a MenuTree.
- Added MenuItem enum to represent an action or a separator in a MenuTree.
- Added menu example.
- Moved Modifier enum and KeyBind struct to libcosmic.
- Moved menu_button macro to libcosmic.

src/widget/menu.rs

@@ -54,7 +54,9 @@
 //! ```
 //!
 
+pub mod action;
 mod flex;
+pub mod key_bind;
 pub mod menu_bar;
 mod menu_inner;
 pub mod menu_tree;