Some extra checks for finding unused display.

Along with some more lambda goodness.
Seat: Add destructor that stops seat automatically.
Log message when daemon stopped.
Seat: remove displays on stop.
Fix shutdown when a user session is active.

Single character fix that took my hours...
Use a more sensible way of managing displays.

Instead of stopping with a flag to restart, start the display when user
session ended or display server somehow stopped.

Stop display before restarting.
Only restart display when that is the only one.
o.fd.DisplayManager: Implement seats property.
Remove unused method.
o.fd.DisplayManager: Implement sessions property for seat and manager.
Conver methods for dbus interfaces to pascal-case.
No need for a member.
qApp like daemonApp.
Rename SessionProcess -> Session.
Remove unused forward declaration.
Remove extra semicolon.
Clean up.
Call quit directly when signal received.
Cosmetic fixes.
Return correct capabilities even in test mode.