Please test if this has any regressions

Please test if this has any regressions

* include/codecs.h:
  - created a subtitle_data_t to be used by subtitle demuxers
    and decoders to pass information.
  - ToDo: access/dvd/es.c and spudec need to be fixed to use the palette field
    of this new struct.
* modules/codec/subsdec.c:
  - moved the decoding of ssa textlines to here.
  - ToDo: support for any tags is lacking atm., but now possible.
* modules/demux/mkv.cpp:
  - ssa is now passed undecoded to ssa subsdec.
  - ssa headers are passed to ssa subsdec via subtitle_data_t
  - ToDo: decode idx header info and fill the subtitle_data_t
  - ToDo: support for compressed vobsubs.
* modules/demux/util/sub.?:
  - moved ssa decoding out of here.
  - ToDo: add support for multiple tracks
  - ToDo: implement reading vobsub .sub files (not .idx)