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

...rather than when disabling it. Otherwise service discovery plugins
cannot possibly cleanup after themselves (and CABOOM).

Whatever would need this would be a buggy SD plugin.

A destructor would never have to release its own object, since the
destructor is only called when the object is not referenced anymore.
On the other hand, it is not clear to me whether SD objects still need
a destructor.