sharing: Start gnome-remote-desktop --handover when gnome-remote-desktop --system is running

In order to provide a login screen over gnome-remote-desktop, their needs to be both system and session instance of gnome-remote-desktop. The latter should get run if the former is running, and not otherwise.

This commit adds a mechanism for doing this by introducing the concept of "assigned services" (versus the already existing configurable services).

Assigned services don't start based on GSettings configuration, but instead just start if an associated system service is running.