[block-tune] Html returned by render() is not searchable

Noticed that the html returned from the `render()` method of my block-tune i**s not searchable** via the `cdx-search-field` input (it is rendered inside `ce-popover__custom-content`).

But if I return a [menu configuration](https://editorjs.io/menu-config/) from `render()`, then the item created **is searchable** (it is rendered inside `ce-popover__items`).

We should be able to chose the zone of the settings area in which our item are rendered (`ce-popover__custom-content` or `ce-popover__items`) and make it searchable or not.