MCF format: Support for modified audio/video/subtitles rather than simple deletion

Sometimes it may be sufficient to modify audio, subtitles, and/or video. If the film is not in an understood language, for example, it can be trivial to change the words slightly rather than muting. Perhaps for video modifications, some subset of the ASS format can be used (or maybe move MCF to ASS entirely? but that's a big change)

The issue discusses the MCF format and possibly using a subset of ASS, but names no files, tests, or entry points. First determine where MCF parsing and media modifications are implemented and whether ASS is already supported. Done would require an agreed format and clearly defined behavior for modified audio, subtitles, and video.

Written by the indexing model from the issue text.