Made SLIC helper collapsed if not available
Make conversion functions generic

Before integer label image to RGB label image to GIMP layer conversion
assumed and operated on member variables that contained these images.
Now the functions simple do the conversion, taking input and spitting
output. This is in preparation for multiple layer support.
Restructured directories a bit.
Added multiple label layer support.

Added app_dirs.py (see https://github.com/ActiveState/appdirs).
Tweaked README and added some clarifications.
Fixed bug with label picking causing an exception and failing to display
label name.
Added "Delete Label" and "Select Mask" helpers.
Added previous/next buttons.
Fixed unsorted directory listing.