Add a virtual function QWindowPrivate::closestAcceptableGeometry()
which is called from the platform plugin.

Task-number: QTBUG-36220
Task-number: QTBUG-36318
Change-Id: I2b3d205e2c75f1d4dd2ba1d333b0d89bc0fcf13a
Reviewed-by: Jan Arve Sæther <jan-arve.saether@theqtcompany.com>