Don't send button events to surfaces when dragging or resizing

It turns out sending button events during all seat operations is not
desirable. This patch introduces a new property
`seatop_impl.allows_events` which allows each operation to define
whether button events should be passed to the surface or not.

The `down` seat operation is the only one that supports this. As all the
other seatops don't support it, the calls to seat_pointer_notify_button
prior to starting them have been removed.

sway/input/seatop_down.c

@@ -58,6 +58,7 @@ static const struct sway_seatop_impl seatop_impl = {
 	.finish = handle_finish,
 	.abort = handle_abort,
 	.unref = handle_unref,
+	.allows_events = true,
 };
 
 void seatop_begin_down(struct sway_seat *seat, struct sway_container *con,