keyboard: simplify key state handling

We used to store both `pressed` keys and `bound` keys and derive
`pressed_sent` keys from them, which are notified to the client when
switching focus. But I think this was overengineered and we can remove
`pressed` keys and `bound` keys. Instead we now directly modify
`pressed_sent` keys just before calling `wlr_seat_keyboard_notify_key()`.

Technically we could even remove `pressed_sent` keys as they are
duplicated in wlr_seat->keyboard_state.keyboard->keycodes, but we keep
`pressed_sent` keys as it is easier to access.

The only place which required `bound` keys was `handle_modifiers()`.
This function checked `bound` keys to keep widnow switcher alive when
Alt-Tab is pressed and only the Alt modifier is released. The window
switcher is then finished when Tab key is released. I replaced the check
with `(seat->keyboard_group->keyboard.num_keycodes > 0)`. This slightly
changes the behavior; when Alt-Tab and any other key e.g. X are pressed
and Tab key and Alt modifier are released, the window switcher is now
kept alive. But I don't think this breaks any workflows for users.

include/labwc.h

@@ -83,6 +83,9 @@ struct seat {
 
 	struct lab_set bound_buttons;
 
+	/* Keys that have been pressed and not consumed by compositor actions */
+	struct lab_set pressed_sent_keys;
+
 	struct {
 		bool active;
 		struct {