systemActions: Call into gnome-session for suspend

systemActions: Call into gnome-session for suspend

gnome-shell previously talked to logind directly on its own. This,
however, will lead to code duplication: the various inhibitor handling
bugfixes I've made to gnome-session (i.e. handling the new return values
from CanPowerOff/CanReboot/CanSuspend) would have to be copied into
gnome-shell. So instead, this commit just moves the responsibility for
handling suspend into gnome-session.

There's a corresponding change in gnome-session

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4040>