Make mouse::Button::Other take u16 instead of u8

On wayland keys correspond to <input-event-codes.h>,
and they are past the limit of u8, causing the
back and forward buttons to be 20 and 19 which definitely isn't right
(they should all be around 0x110..=0x117).

core/src/mouse/button.rs

@@ -11,5 +11,5 @@ pub enum Button {
     Middle,
 
     /// Some other button.
-    Other(u8),
+    Other(u16),
 }