#3 prevent saving of Transformation and TransformationRule instances with wrong name

Transformation instances have to start with T, while TransformationRule instances have to start with R. This should be checked by the backend.

Potentially, we could allow any metamodel to define a constraint on the filename (probably as a regex).

Added two buttons to the TransformationEditor toolbar to create a new rule and a new transformation. These make sure the name is proper.