JA2 Stracciatella Flatpak

A flatpak manifest for JA2 Stracciatella.

Version bump process

1. Add the new version to io.github.ja2_stracciatella.JA2-Stracciatella.appdata.xml in the releases element.

2. Update the source URL and its associated sha256 checksum in the ja2-stracciatella module.

3. Bump dependent modules when required, by updating the source URL for dependency modules (sol2, fltk, string_theory, etc.).

4. Update Cargo dependencies:

   1. Clone JA2 and switch to the tag corresponding to the version.
   2. Get the flatpak builder tools.
   3. Run python3 flatpak-cargo-generator.py ${JA2_CLONE}/rust/Cargo.lock -o cargo-sources.json

5. Commit everything into a new branch and make a merge request.