libweston: mitigate race when destroying outputs

With some displays connect, disconnect, connect events can happen is a very
short amount of time. When this happens, the output global may already be
destroyed when a client tries to bind it. As a result, the client is
disconnected with a protocol error. See [1] for more details on the general
problem.

To mitigate this problem call wl_global_remove() first and call
wl_global_destroy() several seconds later. This is inspired by the
implementation for the same problem in wlroots.

[1] wayland/wayland#10



Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>