Fix transformer property change check.

We must always go through all transformers, even if we already found one
that changed, because the ones that have changed must be invalidated
individually.

Change-Id: I9825eae5c77730febe3335aa8b9c5b028f427365
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Observe access to project properties in prepare scripts and commands.

And use the stored information in change tracking.

Change-Id: I7f9be7798a1d93acc3694a25c7ba0fc92cdc66b0
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>