Allow the Linux CI to produce AppImages

[it-is-final]

Adds the necessary files for the building of the AppImage (.desktop and the icon files) and makes changes to the CI workflow that allows it to produce AppImages via linuxdeploy (though the workflow retains its ability to produce a non-AppImage binary, and still uses its tar.gz for releases).

The AppStream, .desktop and icon files also allow the Flatpak request (described in #393) to be closer to fulfilled, with only the change to the main.cpp still needed to be done.

[Admiral-Fish]

Does it make sense to continue to provide just the original binary as well unpackaged?

[it-is-final]

It's largely done under the principle of providing choice, but technically there is no need to keep the original binary unpackaged as the binary can still be built through following the build instructions or extracted from the AppImage by running the --appimage-extract option on it through Terminal. But I can change that if it's fine to distribute only the AppImage.

[it-is-final]

So if providing the binary is considered superfluous, here is some changes I could do to ubuntu.yaml.

Remove this section

      - name: Package PokeFinder binary
        run: |
          mkdir upload
          mv build/Source/PokeFinder .
          tar czf PokeFinder-linux.tar.gz PokeFinder
      
      - uses: actions/upload-artifact@v4
        with:
          name: PokeFinder-linux
          path: PokeFinder

After the second actions/upload-artifact@v4 (which would probably be renamed), I will need to add the following:

      # Some shells don't seem to handle that 'é' so should probably add
      # an `mv` command for renaming
      - Name: Package PokeFinder
        run: |
          tar czf PokeFinder-linux.tar.gz PokéFinder-x86_64.AppImage

This will change the PokeFinder Linux releases so that the .tar.gz files released are the AppImages rather than the raw binaries. This would also change the download size to around 30MB which is unfortunate but is in line with the Mac OS, and Windows releases which are all around 15-25MB downlaods.

[it-is-final]

Another small pedantic detail about this PR, it uses linuxdeploy/linuxdeploy rather than using probonopd/linuxdeployqt largely because linuxdeployqt requires building with a really old version of Ubuntu for some reason (apparently compatibility with really old systems).