Util\Translator: show correct language list

List was showing useless stuff such as LC_MESSAGES and no language
but the default one. We should have a look whether LC_MESSAGES is
really needed, it looks so ugly.
FilterWidget: POST instead of GET

Might have been a JS issue, but instead of messing with the URL just
POSTing is easier here - and works. Filters from the search box are
now added flawlessly to the filter tree.
FilterEditor: fix buggy dropdown boxes

There were some issues with option names. Also cleaned up HTML a
little bit and added autosubmission.
FilterQueryString: fix issues with not

NOT is a "chain" with just one subfilter and therefore behaves a little
bit different. Should work fine now.

fixes #6554