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/action.rs

@@ -0,0 +1,7 @@
+use crate::widget::segmented_button::Entity;
+
+pub trait MenuAction: Clone + Copy + Eq + PartialEq {
+    type Message;
+
+    fn message(&self, entity: Option<Entity>) -> Self::Message;
+}