Document and use UserEvent::nullWindow for event delivery

Document and use UserEvent::nullWindow for event delivery

nullWindow is used to track if the window was null when the event
was created. If the window was null at contruction time, the qpa
event delivery code will choose a window to deliver it to.

If the window was not-null at creation time but became null during
the actuall event processing time, the event is ignored.

All the event delivery except the touch event delivery has been
changed to reflect this. I have not changed touch event delivery
because I don't really know what all need to be retranslated into
new window co-ordinates.

Change-Id: Ic62d12a13e70dd4d01226509411817db1b6af108
Reviewed-by: Laszlo Agocs <laszlo.p.agocs@nokia.com>