Improve Other Releases menu

1. Make sorting work
2. Change sequence to reflect importance of matching.
(Note: Not yet implemented, but it would be better if we matched
releases with same number of tracks as Album first.)
3. Add field heading to menu.
Prioritise releases...

... which have correct number of tracks, and match preferred release
countries and formats.
Bug fixes

Handle missing data correctly.

Handle unset preferred releases countries and formats

Move priority handling from releasegroup to itemviews so that if there
are two albums from same Release Group, then the list is displayed
correctly for each.
Fix indentation error

Not sure how this crept in - but only causes an issue if no areas.
I18n of heading line.
Reverting change to countries.py
Translate heading strings
Add NEWS
Address Bitmap comments.
Merge pull request #226 from Sophist-UK/sophist_improve_other_versions

Improve Other Releases menu