Restructure the generation of `_Mixin` classes and `@property` methods

[ohmayr]

We want to generate _Mixin classes and @property methods into separate macros so that _Method and _Mixin classes can be defined all together and the @property methods for each can be defined after the class definitions. This can be achieved by separating out the genration of _Mixin classes and @property methods so that they can be called in separate for loops.