vlc: with vlc 2.x, use vlcMedia for libvlc_MediaMetaChanged event

As reported by Wolfgang, the libVlc 2.x fix applied at
402cbee5e675 ("vlc: fix support for vlc 2.x") still have issues,
causing some troubles at least on audio CDs.

The root cause seems to be that, on vlc 2.x, the event
libvlc_MediaMetaChanged should be registered against the
vlcMedia instance, instead of vlcMediaPlayer instance.

Document that inside the code, as this is not obvious.

While the old way is still supported on vlc 3.x, I opted to
keep the code inside #ifs, as some day we'll drop support for
vlc 2.x, making the code simpler.

Also, since this event is registered aganst a vlcMedia object
and behaves different than when applied against a vlcMediaPlayer
object, I'm also using a different name there, in order to avoid
confusion.

BUG: 407153

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>