Improve startup of bindings using QQuickScreen

Improve startup of bindings using QQuickScreen

We can optimistically initialize the underlying QScreen to the primary
screen and this way ensure that in the common case the evaluation of
bindings using the screen's attached properties results in correct calculations
on startup. This way we can avoid re-evaluating the bindings again later.

Previously on startup all the returned values were zero and the bindings got
evaluated again once a window was assigned.

Change-Id: I98ba5905953f0b5054d924919239d178570250d3
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>