Garbage collection on db data and thumbnails

Add --gc command line option, for interactive garbage collection.
path-split: handle \ as directory separator on windows
Better GC of thumbnails

Check which thumbnail files don't have a corresponding pic file.
Compare files without extension, since thumbnails are created with a
web-compatible format (example a pic file named foo.tiff generates a
foo.jpg thumbnail).
Print call chain and error message on GC exceptions