Make flushWindowSystemEvents return ev. accepted

flushWindowSystemEvents() now returns whether the
most recent event added to the queue was accepted
by Qt or not.

Use QAtomicInt to store the accepted state in order
to avoid a data race on it between the Gui thread
and the event poster thread.

Change-Id: I6c111fdaecda5c514307ca0749a54075fe8e872f
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>