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.
ptr_vector hasn't been updated for C++11, so despite being specifically
designed to store pointers to objects it's less safe and not really any
easier to use than a regular vector of unique_ptrs
