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

WIP as we're currently hitting an issue with the arm builds: https://gitlab.apertis.org/infrastructure/apertis-flatdeb-demo/-/jobs/873811

This could be a binfmt configuration issue.

added 6 commits

• 0ab206c9 - Replace demo app with a graphical application
• 1138ca60 - Remove now-unused build files
• ec5b0f24 - Automatically generate suite file and app manifest
• dc75d368 - Rework CI pipeline
• 2939456b - Prepare building for multiple architectures
• 51111f01 - Add builds for arm64 and armhf

Compare with previous version

added 6 commits

• e518dd22 - Replace demo app with a graphical application
• ef6b265b - Remove now-unused build files
• 420ac785 - Automatically generate suite file and application manifest
• f08e085b - Rework CI pipeline
• a2e7408c - Prepare building for multiple architectures
• 8b5db7d5 - Add builds for arm64 and armhf

Compare with previous version

marked the checklist item update CI pipeline to build the demo flatpak for amd64, arm64 and armhf architectures as completed

marked this merge request as ready

requested review from @fdanis

changed the description

merged