ScreenShield: set LockedHint property from systemd

Logind recently got support for a hint property in Session Object to
inform if session is Locked or not. It is up to desktop environments
to keep this property up to date.

https://bugzilla.gnome.org/show_bug.cgi?id=764773

js/misc/loginManager.js

@@ -40,6 +40,9 @@ const SystemdLoginSessionIface = '<node> \
 <signal name="Lock" /> \
 <signal name="Unlock" /> \
 <property name="Active" type="b" access="read" /> \
+<method name="SetLockedHint"> \
+    <arg type="b" direction="in"/> \
+</method> \
 </interface> \
 </node>';