Remove an unused import
Make the usages of enabledSlot more readable
Ignore hidden files and directories based on `show_hidden_files` option.

http://tickets.musicbrainz.org/browse/PICARD-528
Move STYLESHEET_ERROR to parent class as it may be used in any page
Add ignore_regex option to Advanced options

Filepaths matching this regex will be ignored.
util.is_hidden_path(): get rid of normalized parameter
Simplify loop, as suggested by Mineo
Use more specific exception re.error
log.debug -> log.info, as suggested by Mineo
Add missing space before comment