Move ShellDrawingArea to StDrawingArea

It's nicer to have ShellDrawingArea as a St widget so it can
participate more cleanly in CSS styling, such as queuing a redraw
automatically on style changes, and allowing subclasses to use
CSS styling.

https://bugzilla.gnome.org/show_bug.cgi?id=602131

js/ui/altTab.js

@@ -686,7 +686,7 @@ AppSwitcher.prototype = {
         appIcon.actor.reactive = false;
 
         let n = this._arrows.length;
-        let arrow = new Shell.DrawingArea();
+        let arrow = new St.DrawingArea();
         arrow.connect('redraw', Lang.bind(this,
             function (area, texture) {
                 Shell.draw_box_pointer(texture, Shell.PointerDirection.DOWN,