Move abstract attribute up to Classifier

[Matthias Schoettle]

Currently, only a Class can be abstract, however, ImplementationClasses need this ability too. Therefore, the attribute should be pulled up into Classifier.

[Matthias Schoettle]

References #507: Moves abstract property up to Classifier.

→ <<cset ed2f7063f4a4>>

[Matthias Schoettle]

References #507: Fixes compile errors due to changed Literals.

→ <<cset d22f052042e5>>

[Matthias Schoettle]

References #507: Moves view code up to ClassifierView and adds support for ImplementationClasses to be abstract on import.

An ImplementationClass is either abstract or an interface (unlike Java where interfaces are also abstract).

→ <<cset 0e0814091eb5>>