Refactor QQnxWindow

This patch does following things:
* Remove the root window: First window which is created will serve as a root window
* Allow creation of more than one application window (with every app window having
  an own window group) on one screen
* Fixes a bug when reparanting an EGL window

Change-Id: I1afd64a813bc084c0893b958aa191d4a25c32b9d
Reviewed-by: Matt Hoosier <matt.hoosier@garmin.com>
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>