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.
Results in the display seeking to the active line when audio is opened
(if seeking to active line is enabled), and initializes the scrollbar
properly.
Updates #1732.
Simply zero the memory for audio which hasn't been decoded yet, modify
the audio renderer to avoid caching blocks which aren't ready yet, and
add a progress indiciator to the audio display scrollbar.
