avformat/matroskadec: add WebVTT support

WebM files now support inband text tracks, as described in the
following specification:

http://wiki.webmproject.org/webm-metadata/temporal-metadata/webvtt-in-webm

The Matroska demuxer now detects the presence of WebVTT tracks,
synthesizing WebVTT packets (having codec id AV_CODEC_ID_WEBVTT) and
pushing them downstream in the normal way.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>