mkvparser: create generic track objects

Formerly, it was only possible to create instances of
the Track subclasses, VideoTrack and AudioTrack.  However,
we now populate WebM files with WebVTT metadata blocks,
so we must allow for a third kind of track object.

We now enable instances of type Track to be created,
by providing a new factory function, Track::Create, and
making all Track methods non-pure and with a generic

Change-Id: I7d4c965eb566b9fc2f5ceefe1d43723cf8c1e5f0
2 files changed