sessionMode: Add showCalendarEvents property

Add a sessionMode.showCalendarEvents property, which determines
whether the calendar menu should contain an events section or not.

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

js/ui/sessionMode.js

@@ -10,10 +10,12 @@ const DEFAULT_MODE = 'user';
 const _modes = {
     'gdm': { hasOverview: false,
              hasAppMenu: false,
+             showCalendarEvents: false,
              sessionType: Shell.SessionType.GDM },
 
     'user': { hasOverview: true,
               hasAppMenu: true,
+              showCalendarEvents: true,
               sessionType: Shell.SessionType.USER }
 };