Allow to declare if an extension is displayed in application panel or application index by default

The AddApplication UIXP (https://www.xwiki.org/xwiki/bin/view/Documentation/DevGuide/ExtensionPoint/AddApplicationUIX) is documented as "Adds items to the Application Panel or Application Index".
However in practice declaring a new UIX with this UIXP automatically put the application in the application panel: there's no way to declare this application in the application index directly. We should have a parameter for being able to chose this.