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.
Add modes where the borders to add or remove are automatically
calculated rather than forcing the user to do it manually, and hopefully
make it a bit clearer what exactly will happen.
