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
Fix missing QtCore import
Remove unused imports
Remove unused code
decimal(): fix missing parameter `prec` and update tests
`prec` -> `scale`
ClusterEngine.cluster(): remove unused return value
get_cdrom_drives(): code cleanup
File._load(): makes proto matches subclass definitions, add filename parameter
Indentation fix
Unused variable `e` removed
Rewrite loop as expression value is unused
Rewrite test as expression value is unused
Rewrite loop as expression value is unused
Add missing header
coding: UTF-8 -> coding: utf-8, as all other files