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.

examples/menu/Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "menu"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+tracing = "0.1.37"
+tracing-subscriber = "0.3.17"
+tracing-log = "0.2.0"
+
+[dependencies.libcosmic]
+path = "../../"
+default-features = false
+features = ["debug", "winit", "tokio", "xdg-portal"]