Store timestamps of song files

[Neraste]

This will help the feeder to determine if it should re-parse the song files.

[Neraste]

See how Git handles detection of files changes: here. Basically, it checks file stats instead of file content to avoid a costly file read, using system lstat and checking st_mtime among others. The Python os module and path.py library provide such data with respectively os.lstat and Path.lstat. The second part of the documentation details a race condition problem that I am not sure applies to us.