Take product condition into account when change tracking.

If a product gets enabled when it was disabled before or vice versa, that
should be considered a fundamental change. Otherwise some invariants
won't hold later on.
This also gets rid of some special handling.
The associated new autotest spuersedes two existing ones.

Change-Id: I69ed25a7dfd86773f2444b51b372cc48275487b4
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>