VLCIntf now points to a static getter for the interface thread
pointer.

This fixes several problems: During initialization of
VLCMain, [VLCMain sharedInstance] is not ready yet when all other
objects are initialized inside VLCMains constructor. Due to the
way ARC works, the same applies to dealloc (the main shared instance
is nil already). But in both situations, we need VLCIntf for
callback (un)registration and potential logging.

The mac interface relies on static data in any case and does not
support multiple instantiations.