Main menu: When no UIButton focus present Enter key crashes app

Currently this's fixed by overriding processed/captured events by the EventDispatcher parent class, and then inheriting from it:

Line 219 in 49ebc48

def on_key_press(self, symbol: int, modifiers: int) -> None:

Yet consider moving this logic somewhere in a shared module since these button shall be reused for other than the main menu scenes.