Ignore hidden files and directories based on `show_hidden_files` option.

http://tickets.musicbrainz.org/browse/PICARD-528
Add "Ignore files according to show_hidden_files" to NEWS.txt
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.
Add "ignore_regex ..." to NEWS.txt
PICARD-540: return relative paths for shortened filenames
Merge pull request #206 from xlotlu/filenaming

PICARD-540: return relative paths for shortened filenames
Drop useless spaces in button label

It is problematic when it comes to i18n and is useless here.
Regenerate picard.pot
Update README.md

Typo fix
Update about.py

updated copyright dates
Merge pull request #207 from olea/patch-1

Update copyright year in About
Merge pull request #187 from zas/picard528

PICARD-528: ignore file paths if hidden or excluded by regex