Windows: Destroy tablet support before unregistering window classes.

Silences warnings in tests that instantiate QGuiApplication multiple
times.

QSYSTEM: tst_QGuiApplication::removePostedEvents() QApplication::regClass: Registering window class 'TabletDummyWindow' failed. (Class already exists.)
QSYSTEM: tst_QGuiApplication::removePostedEvents() UnregisterClass failed for 'TabletDummyWindow' (Class still has open windows.)

Change-Id: I6af2d38a2debd35f4dc0d48c09244dff022bd6b8
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>