Observe initial screen for fullscreen windows.

Introduce flag to determine "create" state and do
not try to determine the screen from the geometry.

Fixes breakage introduced by
ded4613a

.

Task-number: QTBUG-31166
Task-number: QTBUG-30724

Change-Id: Ibe8df95e0ea84be070fdd06b158a6a10c76d42be
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>