New API for mult-client apps
Manager.GetClient method in 3e1d9bf4 (service: Allow multiple clients on the same connection) and while that made things possible for the Flatpak location portal, it meant us creating redundant clients for existing system apps (e.g gnome-settings-daemon) and we don't want that.
The last patch in this MR changes
GetClient to the old behaviour of reusing existing clients and adds a new method
CreateClient that always creates a new client object.