Generate commands by Change2CommandTransformer instead of ChangeProcessor

[HeikoKlare]

Currently, a ChangeProcessor has to return Commands that can be executed afterwards.

It makes no sense that the transformation developer (the one who defines ChangeProcessors) has to think about creating commands, but just wants to define change propagating routines that are executed correctly.

We have to move the logic for creating commands to the Change2CommandTransforming and let the ChangeProcessor simply define a method that executes a change propagation.