wlr_keyboard_group: introduce enter and leave

Created by: RedSoxFan

Related swaywm/sway#4975 Sway PR: swaywm/sway#5393

This introduces the enter and leave events for wlr_keyboard_group.

The enter event is emitted when a keyboard is added to the group while a key is pressed that is not pressed by any other keyboard in the group. The data is a wl_array of the pressed key codes unique to the keyboard that should now be considered pressed.

Similarly the leave event is emitted when a keyboard is removed from the group while at least one key is pressed that is not pressed by any other keyboard in the group. The data is a wl_array of the pressed key codes unique to the keyboard that should now be considered released.

The purpose of these events are to allow the compositor to update its state to avoid corruption. Additionally, for the leave event, the focused surface may have been notified of a key press for some or all of the key codes and needs to be notified of a key release to avoid state corruption.

These were previously emitted as normal key events, but they are not normal key events. There is no actual key press or release associated with the events. It's purely for state keeping purposes. Emitting them as separate events allows the compositor to handle them differently. Since these are purely for state keeping purposes and are not associated with an actual key being pressed or released, bindings should not be triggered as a result of these events.