When this capability is negoticated by both the client & the agent,
the clipboard grab messages have an associated serial counter.

The serial is reset to 0 upon client connection.

The counter is increment by 1 on each grab message, by both sides.

The sender of the message with the highest serial should be the
clipboard grab owner, and the current session serial should be
updated.

If a lower serial than the current session serial is received, the
grab should be discarded.

Whenever two grabs share the same serial, the one coming from the
client should have a higher priority and the client should gain the
clipboard ownership.

No special treatement is done for the unlikely case of overflowing the
counter. It may temporarily inverse the priority, until both side have
overflown and/or synchronized.

Note: this mechanism isn't aiming at making "the most recent" (as in
time) side gaining the ownership. One side sending subsequent grab
messages earlier will likely take the ownership over a side sending a
single message simultaneously the other way. It only clears the
situation where both side believe that the other is the current
clipboard owner, by having a global ordering and priority in case of
serial conflict.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Victor Toso <victortoso@redhat.com>