PopupMenu: redo keynav using St.FocusManager

Each menu is a focus manager group, but there is also some explicit
focus handling between non-hierarchically-related widgets. Eg, to move
between menus, or from a menubutton into its menu.

https://bugzilla.gnome.org/show_bug.cgi?id=621671

js/ui/appDisplay.js

@@ -501,11 +501,10 @@ AppWellIcon.prototype = {
                 }
             }));
 
-            this._menuManager.addMenu(this._menu, true);
+            this._menuManager.addMenu(this._menu);
         }
 
         this._menu.popup();
-        this._menuManager.grab();
 
         return false;
     },