Unify the values of `MouseButton` and thus remove `Other` variant in
limit possible buttons to 32, which was picked based on platform
capabilities, where 32 is the highest.
For the reference, SDL has identical limit.
When pressing Ctrl+Space, KeyEvent::text_with_all_modifiers would return
\0. That would get included in the text-input text. When an input method
gets activated, the invalid string returning \0 would get sent as
surrounding text, resulting in a Wayland protocol error and crashing the
application.
