This fixes an issue where having a callback set on a specific object
would make an application crash. For instance, having 2 VLC::Instance
objects and calling logSet on one would cause the wrapper
libvlc_instance_t to invoke this callback. If the instance on which
logSet was called gets destroyed, the object wrapping the callback will
be destroyed as well, thus causing the next log callback invocation to
crash.
All callbacks are now shared among objects that wrap the same underlying
libvlc_xxx_t object.
In addition to that, we now ensure the wrapped vlc object gets destroyed
before our callbacks wrappers.