Reload lastScrollPosition on window enter

We need the lastScrollPosition position to calculated scrolling deltas
on scroll events. Since the position is tied to the device and might
have changed while scroll events were send to other applications we need
to reload the value when mouse focus reenters our application.

Task-number: QTBUG-38274
Change-Id: Ic166648f8e7ae486288cbed339a057e3faa1ef2d
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>