Add a mechanism to install / ask for removal of new / removed default Flatpaks on major version rebase
When we update the list of Flatpaks installed by default in the installer, this does not impact the Flatpaks installed on existing systems.
We currently have no way to suggest to users to install new Flatpaks and remove deprecated ones.
We could look at https://github.com/ublue-os/yafti for example, or write a cross desktop solution with a per desktop (GTK/Qt) UI.