Subtitle becomes text.
Then have text content/decoder which can be used-as subtitle or ccap.

Doubling-up of subtitle/ccap starts in player: that emits subtitle or closedcaption, writer writes both.
libdcp gets closedcaptionasset with some common ground with subtitleasset, maybe
add_text_asset() which is called for subtitle + ccap by ReelWriter::create_reel()