Design interfaces needed for multiple sub interpreters using PyObjC

[ronaldoussoren]

When having multiple sub interpreters that use PyObjC sub interpreter A could see Python objects that are created in sub interpreter B. Design a way to deal with this:

• Converting from ObjC to Python should check the owning sub interpreter for the various OC_Python* classes and create a generic proxy when the owner doesn't match the current thread
• Should have proxy registries per sub interpreter, not globally
• ...

NOTE: I'll probably not support sub interpreters in PyObjC because PyObjC changes global state and hence any subinterpreter that uses PyObjC cannot and should not be shut down (in particular not when a new ObjC subclass is defined in the sub interpreter)

[ronaldoussoren]

Ideally this would also have an API that embedders can use (see for example #460). It should be fairly easy to add an ObjC class with a documented interface using class methods that can be used for this. That should isolate embedders from the technical details of this, allowing further evolution of the implementation.

Including documentation and tests of course...