Attach custom classnames to the DOM root of a node instead of nested inside

Another small suggestion for a neater design! Would be fantastic for the onCreateMenu callback to be able to return `undefined` and have the rendering of the context menu icon skipped or set to `display:none`. This way the tree view can more easily convey at a glance which of the elements may have context actions for those of us that are replacing the menu with our own actions.
```
JSONEditorOptions.onCreateMenu?: ((menuItems: MenuItem[], node: MenuItemNode) => MenuItem[]) | undefined
=>
JSONEditorOptions.onCreateMenu?: ((menuItems: MenuItem[], node: MenuItemNode) => MenuItem[] | undefined) | undefined
```
Alternatively, an isEmpty check of the MenuItem array would do the same job.

Cheers!