Line folds are managed using metadata of AssDialogue elements, and
saved in the project properties. They only affect the appearance of the
subtitle grid, and have no impact on other line operations.
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.
