Moving an audio folder causes tracks to be removed from library

[jrha]

Previously, if a folder was moved, the player would remove all the tracks from that folder from the library when attempting to play them, this "feature" was removed in db38090 but clean_library.py is still capable of nuking everything from orbit.
We should provide a --dryrun option and/or backup the library before cleaning.
It would also be nice if ./update_library was capable of detecting tracks that have been moved so that stats can be preserved.