Add keyboard key-repeat support

[waterisbluexd]

handle_repeat_info() stores rate/delay from wl_keyboard::RepeatInfo, but nothing ever uses them handle_key() only forwards one event per physical press, so holding backsapce does nothing until you release and press again.

This adds a calloop timer, started on key down using the real reported rate/delay, that refires the key press until release or focus loss cancels it, tested ot locally holding Backspace/letters in a TextInput