TCS cleanup

All tasks that we find that should be changed in the TCS metamodel and concrete syntax should go here.

Use common superclass for things that can contain a Sequence instead of having different ones. Currently a multitude of references exist for this making it difficult to navigate to a parent of a sequence. Still a problem could be that ConditionalElement e.g., has two sequences (thenSequence and elseSequence). How would this case be handled?
Use direct reference from TextBlock to chosen alternatives in the respective Templates. This makes computation on which alternatives were chosen during parsing a lot easier. As also conditional elements need to be handled in this way, the TextBlock could also reference Sequence directly to also be able to reference a thenSequence and an elseSequence the same way.
Use direct references to OCLExpression in constructs using OCL. Use an the OCL synax written in FURCAS for that.
Remove refersTo and the other related property arguments (create, lookIn, createIn). TCS keywords seem to be the last place where refersTo is still used instead of the new referenceBy. We could use a forEach to create the these keywords.