Closed Signing is not represented

[JuergenGrupp]

Currently, only closed Signing is represented as a Subclass of a VideoTrack.
Can Signing be open, i.e. burnt into the video material? If so, it needs to be represented.
Here's my proposal:

• remove current subclass Signing from VideoTrack
• create Signing as subclass of Track (as with Captioning or Subtitling) and
• create OpenSigning and ClosedSigning as subclasses of Signing (similar to OpenSubtitling and ClosedSubtitling as subclasses of Subtitling)

OpenSigning can then be linked with hasTrack to a VideoTrack.

Comments are welcome!