Write baseclasses in C++ to be able to add methods

[FRidh]

Write the baseclasses in C++ by subclassing App::FeaturePython.
This opens the possibility to add methods to the classes. When this is done, most actions can be put in the baseclasses.

[FRidh]

Not necessary anymore as it is possible to add methods to the adapter classes. See commit d701483

[FRidh]

Methods are not stored when saving.

By writing ABC's in C++ and using pointers to Python methods it should work.