Store the data in vectors rather than AssFiles since even an intrusive
linked list has comically high memory overhead. Cuts memory usage of a
full undo stack with 15k lines by 65 MB for 32-bit and 130 MB for
64-bit. Also roughly halves how long it takes to copy the file for the
undo stack, and makes undo/redo a bit faster.
Relying on TextFileReader to do the charset detection results in the
user being prompted to pick a charset twice when it can't be
auto-detected, since the result from trying to open the subtitles as
timecodes was not being reused.
Closes#1512.
Add SubsController, which deals with things like what subtitle file is
currently open, rather than the contents of the current subtitle file.
Move the rest of the relevant logic from FrameMain there in addition to
all of the stuff from AssFile.
