When a realm has enabled 'use-flatpak' a .desktop file for GNOME
Software will be automatically generated while that realm is running.
This .desktop file will launch GNOME Software from Citadel inside a
bubblewrap sandbox. The sandbox has been prepared so that GNOME
Software will install flatpak applications into a directory that belongs
to the realm associated with the .desktop file.
When a realm has enabled 'use-flatpak' this directory will be bind
mounted (read-only) into the root filesystem of the realm so that
applications installed by GNOME Software are visible and can be launched.
Change the way citadel-desktop-sync works so that gnome-shell
can be aware of applications from several realms at once.
Each application id is now placed in a separate namespace by
adding a prefix to the desktop filename.
