Use attributes.pot from musicbrainz server to generate up-to-date cover art types list 

This is the same principle we used for countries:
* retreive and parse a pot file
* generate a python file from it
* use corresponding .po files to ensure translations, reducing translations redundancy between mb server and picard

The thing is a bit hacky here, because python gettext currently fails to handle msgctxt which are used in attributes.pot