sessionMode: Add allowSettings property

Add a sessionMode.allowSettings property, which determines whether
menus in the top bar should allow access to System Settings or not.

https://bugzilla.gnome.org/show_bug.cgi?id=676156

js/ui/sessionMode.js

@@ -11,11 +11,13 @@ const _modes = {
     'gdm': { hasOverview: false,
              hasAppMenu: false,
              showCalendarEvents: false,
+             allowSettings: false,
              sessionType: Shell.SessionType.GDM },
 
     'user': { hasOverview: true,
               hasAppMenu: true,
               showCalendarEvents: true,
+              allowSettings: true,
               sessionType: Shell.SessionType.USER }
 };