Appdata related changes

appdata: Mark release descriptions as untranslatable

GNOME automatically excludes release descriptions on Damned Lies (GNOME Translation Platform). It's a good practice to follow the GNOME way.

This can streamline the translation process, allowing translators to focus their efforts on more critical and user-facing aspects of the application.

appdata: Add vcs-browser URL

Add vcs-browser URL to show the source code repository.

appdata: translate=no properties

It appears that the appstream project no longer supports translatable=no properties, and gettext extract them as translatable.

I opened an issue to inform about the situation, but translatable=no properties are not accepted by developers. You can find the issue here: https://github.com/ximion/appstream/issues/623

Please test your script or string extraction process before merging this PR.

In MetaInfo files, each individual paragraph of a description (or enumerated entry) is translated individually, however, you can only exclude the complete block from being translated by adding translate="no" to the description element.

Source: https://freedesktop.org/software/appstream/docs/sect-Quickstart-Translation.html