It doesn't feel like Flash without having the hand cursor display
when hovering over buttons. First pass at implementing this;
core communicates which mouse cursor to use via
`InputBackend::set_mouse_cursor`.
TODO: Hand cursor only displayed for Button display objects
currently. Movie clips should also display this when they are in
"button mode" (when a button mouse event is set on them in AVM1,
or `buttonMode` property in AVM2).
Nathan Adams