Improved support for advanced archives

Now has correct counts in archive info, and displays cover counts as
well.

Renamed a duplicate (import_failed) string.
Advanced backup now done

Not all features implemented: preferences hard-code always selected;
option to specify a custom date is hidden/removed (though supported
internally).
Ensure we don't export blank author or title fields
Fix heading when in "Search by name" mode.

Issue 538
Initial work on release changes
Translation changes and string cleanups