Fix drag & drop in playlist layout editor, filter editor etc. in non-english locales

The implementation relies on custom mime-type specified in .ui files.
However, the strings weren't marked as non-translatable, so language
teams that decided to translate application/x-amarok-tag-token effectively
broke drag & drop for their users. (the blame goes to Amarok team, of
course)

Myriam, more than 2-year-old bug fixed in a couple of hours, don't you
like your GSoC student? :-)

BUGFIXES:
 * Fix drag & drop in playlist layout editor, filename layout editor,
   filter editor for non-english locales. (BR 215402)

BUG: 215402
FIXED-IN: 2.6