Cleanup playlistfolder.
Cleanup most of playlistcontainer.

Missing bit is mainly the linked list handling of callbacks.  Also removed what
looked like a very bad use of Py_FreeMem that would essentially free the
internal string pointer from a python string (if I understood the docs
correct).

This starts using PyObject_CallFunction in the callbacks, as that way we any
NULL objects will be caught without us needing to handle them ourselves all
over. This also means any objects going in as format "N" need to be XDECREFed
not DECREFed as they could easily be NULL.