EventManager: Fix potential use-after-free

When move-assigning, or copy-assigning, any object that holds an event
manager & inherits from Internal will have the internal class
copy/move constructed first, causing its internal shared pointer to
be overriden, and in turn the libvlc object to be released.
This is a problem if this object contains an event manager with
registered events, as the event manager will be overriden afterward,
causing its destructor to unregister the events after the libvlc
object it needs to unregister the events has been released.

This commit ensures we hold on to the object associated with the event
manager, and that we explicitely unregister all events before releasing
the object.