Notify user that the library has to be rescanned

This is necessary so that we can display cover-art for users that are
upgrading.
Clean up code

Coding style changes and removal of dead code.
LateNight: Comment out coverart show/hide connection since there's no control to trigger it
FIX 1390415: Parse Cover for opus files

This adds cover parsing for opus files and a test as well.
Rework update notification text
Merge pull request #370 from kain88-de/ask-for-rescan-upgrade

Ask for rescan upgrade
Fix loop tests so the loop sizes are larger
Only connect if non-NULL.
Add TaskWatcher utility class.
Move directory blacklist creation to a utility method.
Remove unapplied argument to CoverInfo operator<<
People seem to think 30 samples is a good number
Update translation template. Found 1812 source text(s) (9 new and 1803 already existing).
Move LibraryHash/track_locations cleanup code from ~LibraryScanner to TrackDAO::finish.
Signal to the main-thread TrackDAO rather than touching it directly from
the LibraryScanner.
Fix reload-cover-art connection in DlgTrackInfo.
code style hints
Merge branch 'master' of git://github.com/mixxxdj/mixxx into starrating_widget