It's modestly faster, significantly more type-safe, and doesn't assert
when there's too few arguments, which causes problems for plural forms.
Closes#1733.
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.
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.
