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/overview.js

@@ -99,10 +99,8 @@ const ShellInfo = new Lang.Class({
 const Overview = new Lang.Class({
     Name: 'Overview',
 
-    _init : function(params) {
-        params = Params.parse(params, { isDummy: false });
-
-        this.isDummy = params.isDummy;
+    _init : function() {
+        this.isDummy = !Main.sessionMode.hasOverview;
 
         // We only have an overview in user sessions, so
         // create a dummy overview in other cases