Input manager now emits signal for album art and gives
 url to art itself, so CoverArt or Background-widgets don't
 need to parse it themself, and CoverArt don't need to use
 those callbacks. This ways both get same image too.

Signed-off-by: Christophe Mutricy <xtophe@videolan.org>

At the moment, we assume RTP/RTCP multiplexing anyhow.

 * Make registration thread-safe, and independent on vlc_list_find().
 * Fix use-after-free race condition when destroying the interface
   (interaction will be lost instead).

Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>

Currently, we stick to legacy application/ogg for legacy extensions.

but take only the first one.

Initial patch by Hannes Domani with the comments:
"Hello

 this fixes the problem when font is not attached in mkv.

 was mentioned in:
 http://forum.videolan.org/viewtopic.php?f=14&t=51096&p=166340
 http://forum.videolan.org/viewtopic.php?f=7&t=50534&p=163900
"

I used customEvent to implement it.
(It renders the dedicated message thread per subscriber useless with qt4).

Otherwise, we would have to either ignore errors, or fail to load some
plugins just because earlier ones failed