Set SDK to macOS Big Sur.

This is a couple of commits above 0.2.0 to include the environment
setup for macOS.

I can't support what I don't use.

Lose the acknowledgements and add gtk-osx in the license section.

Clearly state that this isn't a fork, Gitte's code isn't here.

Use ci_gitlab_add_metadata function.

The issue was two-fold:
  - Gitte was using a build-time setting to find the locale directory.
    This was fixed upstream and is the reason the version was updated.
  - Gitte ships very few (2) localizations at the moment, which causes
    the application bundler to only put these two into the plist. This
    apparently has funny side effects.