output.done event can be sent even on some property change, not only
when announcing the output. Therefore we must check if we already have it
otherwise we may corrupt the list by adding it multiple times.

This fixes bug when xwayland looped indefinitely in output.done handler
and that can be reproduced following these steps (under X without
multi-monitor setup):
 1) run weston --output-count=2
 2) run xterm, move it so that half is on one output
    and half on the other
 3) close second output, try run weston-terminal

weston sends updated outputs which trigger this bug.

v2. factor out common code into function
    move expecting_events into right branch

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>