The only event that lock_and_wait can only wait for the end of the
interface event (safely). That is the only piece of state which is read
under the lock.

Also it is not possible to wait on a random object since (deprecated)
vlc_object_signal() is _not_ broadcasting the object condition variable.
In effect, VLC threads shall only wait on their own object (this is not
LUA-specific).

Ultimately, lock_and_wait is of barely any use, and in any case is now
poorly named (my fault for inventing vlc_object_lock_and_wait()). By the
way, it always returns true now.