On Wayland, don't drop extra mouse buttons

This commit forwards "unknown" Wayland mouse buttons downstream via
'MouseButton::Other'. Possible values for those could be found in
<linux/input-event-codes.h>.

Also, since Wayland just forwards buttons from the kernel, which are
'u16', we must adjust 'MouseButton::Other' to take 'u16' instead of
'u8'.

src/event.rs

@@ -745,7 +745,7 @@ pub enum MouseButton {
     Left,
     Right,
     Middle,
-    Other(u8),
+    Other(u16),
 }
 
 /// Describes a difference in the mouse scroll wheel state.