keyboard: make keybind match stricter

...and avoid failing to send release events to clients for any keys that were
not absorbed by a keybind.

Do not match keybinds if there are other non-modifier keys (not part of any
defined bind) pressed at the same time.

Only store non-modifier keycodes in the key-state.c 'pressed' array.
This makes the call to wlr_seat_keyboard_notify_enter() in seat_focus()
consistent with the equivalent in sway (in seat_keyboard_notify_enter()).

Fixes: issue #1091

src/key-state.c

@@ -102,3 +102,9 @@ key_state_nr_keys(void)
 {
 	return bound.nr_keys;
 }
+
+int
+key_state_nr_pressed_keys(void)
+{
+	return pressed.nr_keys;
+}