Allow generic plugins to set defaults for window system properties

Allow generic plugins to set defaults for window system properties

In order for generic plugins to set defaults on "startup" time - such
as the meego integration plugin to set the correct screen orientation -
it is necessary to construct the plugins when the application startup is
done. Then the plugin can "inject" the values the usual way, using
QWindowSystemInterface::handle*Change. Afterwards we need to process those
events - take them from the window system event queue and let QGuiApplication
process them.

Change-Id: I84de022ad565a33ae3ef5dfc34f540d6bf488b03
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>