Is your feature request related to a problem? Please describe

Currently, the systemd config file makes it always start with the session. We can make it run only when it is required

Describe the solution you'd like

Systemd allows to start a service when another component requires its D-Bus service. It would be great to have touchegg only started by systemd when a component (like the compositor) is using it.

Additional context

This has been requested by Fedora maintainer in order to not have touchegg run on a pure GNOME session when installed for the Pantheon session.