Change "FilePath" type to "AudioFilePath"
Add Gramophone.scanDirectoryForAudioFiles

Non-recursively scans a directory for possible audio files,
based on file globs. (Currently *.flac, *.m4a and *.mp3)
Add scanTreeForAudioFiles function

New function recursively scans a directory for audio files, based
on file extension. This function is very slow (and has very poor
memory usage) for large trees; it needs to be re-written to return
files one at a time, along with some indicator of progress, and
allow interruption.