Now that we use hashes to identify notes, a garbage collector comes in
handy. We search for note files that aren't referenced anywhere. Such
files might come up if a note, that isn't connected with any other item,
is edited.
Huge parts of this code are very hackish due to our data structure
layout and the way our hash table implementation works.
Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>