Implement synchronous WS events by flushing.

Implement synchronous WS events by flushing.

Previously synchronous window system events were
implemented by bypassing the queue and processing
the event immediately. This is not ideal since the
event order is not preserved - there might be "happened
before" events waiting in the queue.

Add QWindowSystemInterface::flushWindowSystemEvents
and change all handleSynchronous* to 1) queue the
event 2) call flushWindowSystemEvents.

flushWindowSystemEvents is almost identical to the
already existing sendWindowSystemEvents with the
exception that it does not call QApp::sendPostedEvents.
Move the common implementation to a new private function.

Task-number: QTBUG-20778

Change-Id: Ie98a83875bc0a14e335e36bed0dd9e0ed4a1dea0
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>