Fixes errors when trying to automatically open audio from dummy video
and improves UX a little by disabling the Open Audio From Video menu
item when there are no audio tracks.
Most of the dialogs in Aegisub have a public interface consisting of a
single function, so there's really no need to expose the actual dialog
classes to the rest of the program. Add dialogs.h with the declarations
of all of those functions (most of which are just ShowFooDialog()), and
kill a million other headers.
Remove it from the script info section and put it in its own section
that isn't tracked by undo and make it not stringly typed. Removes the
need for the gross hack where changes are slipped in just before saving
to circumvent the undo system, cuts down on the uses of string literals
to identify fields, and probably improves performance a little.
Add a new Project class which is responsible for everything related to
opening and closing audio, video, subtitles, timecodes and keyframes.
This pulls almost everything not directly related to playing audio/video
out of the audio and video controllers, pulls more crap out of
FrameMain, and happens to make things a little simpler in the process.
