Include "Session Services" and "Background Services" entries in the Applications table

This adds the "session.slice" and "background.slice" cgroups as (aggregated) rows called "session services" and "background services". The rows contain all processes in either the session.slice cgroups or the background.slice cgroups. This should result in the applications page being a more comprehensive overview of what's actually running in a session.

It also disables the "Quit" and "Send Signal" actions for these entries, to prevent a user from killing their entire session.

Depends on libksysguard!390 (merged)