Add a radial background shade for modal dialogs

Use a new ShellGLSLQuad actor class to build a RadialEffect that can be
enabled on Lightboxes to achieve a radial effect similar to the overview
one. Then enable it for modal dialogs.

https://bugzilla.gnome.org/show_bug.cgi?id=669798

js/ui/modalDialog.js

@@ -89,7 +89,8 @@ const ModalDialog = new Lang.Class({
 
         if (!this._shellReactive) {
             this._lightbox = new Lightbox.Lightbox(this._group,
-                                                   { inhibitEvents: true });
+                                                   { inhibitEvents: true,
+                                                     radialEffect: true });
             this._lightbox.highlight(this._backgroundBin);
 
             this._eventBlocker = new Clutter.Actor({ reactive: true });