sessionMode: Add hasOverview property

Add a sessionMode.hasOverview property, which determines whether
the mode should give access to the overview or not.

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

js/ui/sessionMode.js

@@ -8,9 +8,11 @@ const Params = imports.misc.params;
 const DEFAULT_MODE = 'user';
 
 const _modes = {
-    'gdm': { sessionType: Shell.SessionType.GDM },
+    'gdm': { hasOverview: false,
+             sessionType: Shell.SessionType.GDM },
 
-    'user': { sessionType: Shell.SessionType.USER }
+    'user': { hasOverview: true,
+              sessionType: Shell.SessionType.USER }
 };
 
 function modeExists(mode) {