Revise AppImage build

[git-developer]

This is not really an issue. I just want to let you know that I ported the AppImage build to appimage-builder, replacing the existing scripts appimage-build.sh and scripts/appimage-AppRun.sh.

My motivation was that the AppImage didn't work in my runtime environment, the retro gaming distribution batocera. Some dependencies were missing, some were outdated, some hardcoded paths didn't match.

I read the discussion about Python 2 and learned that Ryochan7 maintains a Python 3 fork while this repo is heading towards C. My AppImage build is based on Ryochan7's port, so it will probably not work out-of-the-box here, but I think it could work with small modifications.

If this topic is of interest at all, you can find the code in Ryochan7#98. It consists of three parts:

1. A Dockerfile to build the application on any Docker-capable host
2. An AppImageBuilder.yml descriptor to build the AppImage using appimage-builder (which is available as Docker image, too)
3. A GitHub Action workflow that runs these builds with multi-platform support (e.g. amd64 & arm64)

The base OS for the Docker and AppImage build is configurable to Debian-based distros and was successfully tested on Ubuntu Jammy (LTS) and Lunar.