This happens via VapourSynth's message logger.
Also add wrapper functions for this to aegisub_vs.py and add progress
updates everywhere - both to improve UX and to help with debugging for
when scripts get stuck somewhere.
Add a utility library that wraps LWLibavSource and can parse its .lwi
file to obtain timecodes and keyframes. It also contains a function to
generate and save keyframes using WWXD or Scxvid. Update the default
scripts to use these functions.
