lavf: add AV_DISPOSITION flags for WebVTT text track kinds

lavf: add AV_DISPOSITION flags for WebVTT text track kinds

There are 4 separate WebVTT text track kinds: subtitles (the default
if not otherwise specified), captions, descriptions, and metadata.
The WebM muxer needs to know which WebVTT text track kind this is, in
order to synthesize the correct track type and codec id.

To allow a demuxer to indicate the text track kind of the input, a new
set of AV_DISPOSITION flag values has been added, corresponding to
each of the non-default text track kind values.