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.
d16845d5f0