Improve demo runtime and application

This MR improves the current demo by creating a flatpak for a graphical application (gnome-font-viewer), which can be used to demonstrate flatpak running on agl-compositor.

It introduces the following changes:

• add GTK to the runtime and SDK in order to build graphical applications
• replace the current demo app with a graphical one
• add a generate-files.sh script to automatically generate the build files for a given Apertis version
• update CI pipeline to build the demo flatpak for amd64, arm64 and armhf architectures