At least on OS X, allocating the buffers is one of the more expensive
parts of video playback, and on an arbitrary 720p H.264 file with simple
subtitles this cuts CPU usage while playing by about 30%.
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.
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.
