Add MouseButton::{Back, Forward} to MouseInput

Add named variants for physical back and forward keys which could
be found on some mice. The macOS bits may not work on all the
hardware given that apple doesn't directly support such a thing.

Co-authored-by: daxpedda <daxpedda@gmail.com>

src/event.rs

@@ -1202,12 +1202,19 @@ pub enum ElementState {
 }
 
 /// Describes a button of a mouse controller.
+///
+/// ## Platform-specific
+///
+/// **macOS:** `Back` and `Forward` might not work with all hardware.
+/// **Orbital:** `Back` and `Forward` are unsupported due to orbital not supporting them.
 #[derive(Debug, Hash, PartialEq, Eq, Clone, Copy)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 pub enum MouseButton {
     Left,
     Right,
     Middle,
+    Back,
+    Forward,
     Other(u16),
 }