Add QTCREATOR_COMPAT_VERSION

Necessary for keeping compatibility within a minor series.
When bumping versions, the compatVersion now needs to be taken into
account too.

Change-Id: I588947f885480889ff2f58481f72db76e3f1a9f1
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>