keyboard: Implement key repeat for keybindings

It seems that every Wayland client is expected to implement its own
key-repeat logic, rather than doing it server-side as in X11.  This
means that labwc also has to implement its own key-repeat logic for
compositor keybindings.

This is a very simplistic timer-based implementation.  It doesn't
attempt to synthesize accurate timestamps, and may lag depending
on system load, but it appears to get the job done.

v2: Use server->wl_event_loop
v3: Comments and formatting

src/seat.c

@@ -24,6 +24,7 @@ input_device_destroy(struct wl_listener *listener, void *data)
 		struct keyboard *keyboard = (struct keyboard *)input;
 		wl_list_remove(&keyboard->key.link);
 		wl_list_remove(&keyboard->modifier.link);
+		keyboard_cancel_keybind_repeat(keyboard);
 	}
 	free(input);
 }