Flatpak Metadata for PySolFC (WIP)

A Flatpak packaging setup for PySolFC that should be ready for submission.

Additional "Nice to Have" Ideas

1. Check if Tkinter requires any of the .tcl files in /lib or if we can add them to cleanup to reduce the package size.
2. Replace 0001-allow-app-paths.patch with a proper upstream fix to have PyGame derive the install prefix from its location rather than hard-coding the list of places to look for assets.
3. Contact skomoroh for permission to relicense the rest of the release announcements for inclusion in the Appstream XML file.
4. Clean up staleness in PySol's build instructions and deduplicate build automation as noticed in this comment. (May want to setup some Vagrantfiles to automate spinning up the various distros and installing PySolFC to identify staleness in the dependency installation commands given in the README.)
5. Convert take_screenshot.py from a proof of concept to a proper tool.
6. Contribute a Flatpak theme package for the GTK+ 2.x version of Breeze to Flathub (it'd also benefit apps like Geeqie) and then try adding gtkTtk as PySolFC's default Ttk theme... possibly through python-gttk
7. Port the experimental GTK backend from PyGTK to PyGObject so there's an avenue forward for native Wayland support. (Must also add gtk3-nocsd to the build so the Flatpak package knows how to obey when a non-GNOME desktop sets LD_PRELOAD=libgtk3-nocsd.so.0 to prevent a usability regression.)

How to Test Locally

1. Make sure you have Flathub set up with suitably new versions of flatpak and flatpak-builder installed. (The flathub instructions should cover that too)

2. (Optional) Make sure you have the supporting validators installed locally:

   flatpak install flathub org.flathub.flatpak-external-data-checker org.freedesktop.appstream-glib

   (Note that the version of appstream-util from your distro package manager is likely to be an old version that is no longer aligned with current standards for good Appstream XML and may reject valid constructs.)

3. Run the following commands from the root of the repository:

   # -- Optional --
   flatpak run org.freedesktop.appstream-glib validate io.sourceforge.pysolfc.PySolFC.appdata.xml
   flatpak run org.flathub.flatpak-external-data-checker io.sourceforge.pysolfc.PySolFC.json
   # -- Required --
   flatpak-builder --user --install --force-clean build io.sourceforge.pysolfc.PySolFC.json
   flatpak run io.sourceforge.pysolfc.PySolFC

   (If flatpak complains about a missing version of org.freedesktop.Sdk, just flatpak install what it complained about and try again.)

While you're at it, you can also check something else that Flathub wants which is outside the scope of this set of files by installing desktop-file-utils with your system package manager and running desktop-file-validate on the .desktop file. (As of this writing, it raises no issues.)

flatpak-external-data-checker will be run periodically for you by Flathub, but you may want to setup GitHub Actions to verify that commits to your upstream repo haven't broken any .desktop or .appdata.xml files you store in there, since Flathub does want them to be correct but they're not Flatpak-specific.