Hook property setters into Objective-C validators

[Gobbel2000]

Setting an illegal value through the property setters of the Python wrapper should raise a descriptive ValueError (or TypeError). This should be accomplished by calling the appropriate key-value-validation methods in objective C for most setters from within the Python wrapper.

An initial implementation exists in the unit_tests branch at ObjectWrapper/GlyphsApp/init.py with _setValueValidation(), doing only type testing for now. This function should eventually be used in all setters where validation is necessary.