Process screen events in the main thread

Screen events are still read in the screen event thread but are
processed in the main thread to make it possible to support
QAbstractNativeEventFilter for screen events later.

Implementation is similar to the xcb platform plugin.

Change-Id: I7bade3e13e51c6d70bb608727a93bbd3aabc5d47
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>