Pext: new and improved AppImage

This weekend, among other things, I was working on improving the Pext AppImage. Sylvia and I were spending quite some hours on making the Pext AppImages smaller, more efficient and provide improved desktop integration. Also, AppImages are now built using linuxdeploy, the universal AppImage creation tool suite, using the conda plugin to set up everything Python related. That plugin was actually inspired by the experiences I made during all the months it took to make the first AppImage build script.