Improve Structural Weaver: Avoid casting wrong types

[Matthias Schoettle]

The structural weaver has a few issues that were encountered when reusing a concern (Authentication in this case):

1. Invalid cast on ImplementationClass to Class in PreProcessor#createSkeletonClasses line 211 (mapping attributes).
2. Similar problem in PreProcessor#processExtends line 459ff when resolving name conflicts, which might be called on an object that is not a Classifier.
3. OperationsUtil#getType returns null when absolutely no type was found (for several reasons). However, setting null as the return type results in an invalid model. Therefore, it should leave the old type and maybe output a warning in case this is not intended to happen.

[Matthias Schoettle]

References issue #147: Preventing invalid casts by making sure the classes are actually classes.

→ <<cset 929f85f90528>>

[Matthias Schoettle]

Resolved issue #147: If no type is found, a warning is printed and the old type used instead in order to not run into problems when using "null" otherwise.

→ <<cset e5dca02ac77e>>