* ALL: Introduction of a new api for decoders.

* ALL: Introduction of a new api for decoders.
   The final aim of this new api is to make it possible to use the decoders from other modules like the transcoder for instance.
   Only a few decoders have been ported to the new api (a52, libmpeg2, dts, vorbis, theora) so the old api is still supported.

   Don't hold your breath, there is still much work to do before we reach this goal.

* modules/codec/a52.c, libmpeg2., dts.c, vorbis.c, theora.c:
   Converted to the new api.
   Merged the a52 and vorbis packetizers in their respective decoders (removes a lot of code duplication).
   New dts and theora packetizers (merged in their respective decoders).