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.
iterator_to requires that the object be in the list, which is sometimes
not the case when processing a commit which removed the active line or a
selected line. To handle this, add a checked version that returns
Events.end() when it is not in the list rather than crashing.