Address certain annoyances with working with the flatpak runtime
- Fix the display of the runtimes buttons to radio to better convey what's happening.
- Ensure that after switching the cmake reload will not just do it for the root directory. It will happen that it fails for the system setup then when you switch the configure works. You need to have all targets available then.
- Since we implemented this, KDE adopted using .flatpak-manifest.json files, which aren't renedered right now.
- Maintain the instances over executions for faster setup.
- Reduce the environment variable chaos.