app-icon: Use the dash's icon size for dragged icons

As the dash is one of the primary drop targets for dragging application
launchers, it's reasonable to use the dash icon size for app icons'
drag actors, especially with the larger size now used in the application
view.

https://bugzilla.gnome.org/show_bug.cgi?id=639428

js/ui/appDisplay.js

@@ -522,7 +522,7 @@ AppWellIcon.prototype = {
     },
 
     getDragActor: function() {
-        return this.app.create_icon_texture(this.icon.iconSize);
+        return this.app.create_icon_texture(Main.overview.dash.iconSize);
     },
 
     // Returns the original actor that should align with the actor