flatpak2AppImage proof-of-concept

@Anti-Ultimate wrote in https://github.com/flatpak/flatpak/issues/408#issuecomment-262438079

> AppImageKit draws in useless libraries from apt the binary doesn't even need, just because Library Y is in Package X which contains 10 other libraries the application isn't linked to. Those just take up space and are completely useless.
>
> I could copy the libaries into the AppDir manually like I used to do, but this has shown not to work well in the past with newer distros containing newer versions of libraries needed. And the libraries to compile can only be found on a specific distro (namely the one which has the libraries specified in the script).
>
> AppImages compiled against the Flatpak runtime/SDK, now that's something I would like.