Add function for scanning directories for audio files

Work-in-progress for adding functions for scanning recursively
through a directory, searching for audio files to add to the
database.
Fix a typo

A "," (comma) where there should have been a "." (dot) meant
that scanTreeForAudioFiles was recursing into "." directories
and therefore never returning.
Delete dead code

Removed a function definition which had already been commented out
in an earlier commit.
Add printAudioFilenames function

Mostly useful for testing scanTreeForAudioFiles.