When there are multiple sessions, we may get a polkit dialog after
the user confirms the power off dialog. Previously, we didn't handle
this situation very nicely, since we were already in the EndSession
phase when calling PowerOff, and there is no way back to the Running
phase from there, even if the user cancels the polkit dialog.

This commit rearranges things so that we call PowerOff before leaving
the QueryEndSession phase. To prevent the system from going down
right away, we take a delay inhibitor, and listen for the
PrepareForShutdown signal to know when we can safely transition
into EndSession. If the user cancels the polkit dialog, the PowerOff
call fails with an AccessDenied error, which we handle and return
to the Running phase. In order to tell gnome-shell that the session
is back to running, we call a new Close() method on the EndSessionDialog
interface.

All of this only works with systemd. The ConsoleKit implementation
works the same as before.

https://bugzilla.gnome.org/show_bug.cgi?id=688076