[Bf-python] Callback types

[Michael Reimpell]

A wrapper for activate_filesel sounds very reasonable since it neither affects 
existing calls nor does it break the api.

Regarding the multiple scripts problem:
> For a proper solution we can make the callback pointer part of
> the Script struct and pass the script name or pointer to e.g.
> activate_filesel_ex instead of a second callback.
How about another wrapper that exists only within BPy that takes the script 
struct as argument and calls activate_filesel_ex? This would allow to keep 
the clean separation of BPy and the rest of Blender for a little cost of BPy 
c api complicacy and performance.