plugins/ogg: Add support for theora videos

plugins/ogg: Add support for theora videos

This adds support for theora videos, with all the fields being filled
in, possibly with several streams.

For now we link to libtheoradec to do this. However it seems a little
bit overkill linking to libvorbis and libtheora. There can also be other
codecs inside ogg, which we don't currently support.

It would be good (but a little more difficult) to stop using them and
parse the file by ourselves. In this scenario the use of libogg would
remain so we get page/packet separation while doing the codec part.