overview: make dash private

The dash object is currently exposed as a public object.
It's only used outside of the overview for the dash object's
iconSize property though.

This commit makes the dash object private and proxies the dash
iconSize property to the overview.

https://bugzilla.gnome.org/show_bug.cgi?id=657082

js/ui/appDisplay.js

@@ -608,7 +608,7 @@ AppWellIcon.prototype = {
     },
 
     getDragActor: function() {
-        return this.app.create_icon_texture(Main.overview.dash.iconSize);
+        return this.app.create_icon_texture(Main.overview.dashIconSize);
     },
 
     // Returns the original actor that should align with the actor