Subtitles: Clean up the code and reorganize somewhat.

* The code specific to cc608, cc708, and srt is made more
  object-oriented.

* Various external caches of the SubtitleScreen's children list
  (e.g. expiration time, cc708 window number, original widget area)
  are integrated directly into the children list for simplified
  management.

* Method definitions are reordered in the file to more logically match
  declaration order.

* Most of the VB_VBI logs are reduced to the LOG_DEBUG level.

* The subtitle creation pipeline is somewhat more amenable to tweaks
  and performance-related experiments.

Refs #10984