Relay focus in/out events to the offscreen QQuickWindow, and also
make the offscreen window believe it has the focus when the
render window has it.

Task-number: QTBUG-39033
Change-Id: Ib50b134e635833ad3813693ca272f04607c525b8
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>