Add JavaScript API for extending Live Data with menu item and panel

As discussed [in the forum](https://forum.xwiki.org/t/add-a-csv-export-to-live-data-in-xwiki-standard/10174), a JavaScript API for extending Live Data with new menu items and panels should be created. For the concrete use case of adding a panel with an associated menu item, it would be useful if this association was automatic and the menu item would automatically toggle the panel. This issue is about supporting this particular use case with a menu item that toggles a panel with extension-provided content, more advanced use cases might be supported later.