PICARD-2746: localize XDG desktop file

[phw]

Summary

• This is a…
  • Bug fix
  • Feature addition
  • Refactoring
  • Minor / simple change (like a typo)
  • Other
• Describe this change in 1-2 sentences:

Problem

• JIRA ticket (optional): PICARD-2746

The XDG desktop file (used for offering the menu entry on compliant desktop environments, e.g. on Linux and BSD) is currently not localized, but can be.

Solution

• Move the org.musicbrainz.Picard.desktop file to a org.musicbrainz.Picard.desktop.in template file.
• Extract the strings from there into po/appstream using msgfmt as part of regen_appdata_pot_file setup command.
• Added the command build_desktop_file command for setup.py to compile the translated org.musicbrainz.Picard.desktop file from the org.musicbrainz.Picard.desktop.in template.

I decided here to use the same gettext resource for the desktop file as for the AppStream metadata. My reasoning is as follows:

• Both provide metadata about the application
• Both are from related freedesktop specifications
• In both cases the translations are only ever used during build. The .po files never get separately compiled into .mo files and no translations files get installed onto the target system (as all translations become part of the generated file)
• As the desktop file has only 3 short strings to translate it feels like too much overhead to maintain a separate translation resource. Both in the picard source code as well as in Weblate.

[zas]

LGTM