PICARD-597: use named place holders in translatable strings

- modify set_statusbar_message(): drop hacky %s replacement, it shouldn't be needed anyway
- add named arguments echo and translate to it, to cover all needs
- rewrite translatable strings to use named place holders
- fix up some messages, add plural forms if needed

http://tickets.musicbrainz.org/browse/PICARD-597
Merge pull request #309 from zas/PICARD-597

PICARD-597: use named place holders in translatable strings