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/platform_impl/macos/view.rs

@@ -1027,10 +1027,14 @@ fn mouse_button(event: &NSEvent) -> MouseButton {
     // The buttonNumber property only makes sense for the mouse events:
     // NSLeftMouse.../NSRightMouse.../NSOtherMouse...
     // For the other events, it's always set to 0.
+    // MacOS only defines the left, right and middle buttons, 3..=31 are left as generic buttons,
+    // but 3 and 4 are very commonly used as Back and Forward by hardware vendors and applications.
     match event.buttonNumber() {
         0 => MouseButton::Left,
         1 => MouseButton::Right,
         2 => MouseButton::Middle,
+        3 => MouseButton::Back,
+        4 => MouseButton::Forward,
         n => MouseButton::Other(n as u16),
     }
 }