Make it possible to edit blocks without DOM node

[dulnan]

In some cases block components can't render anything (e.g. rendering a dynamic teaser list where the query yields 0 results). Currently, they have to conditionally not render only when isEditing is false. A better approach would be to provide a built-in fallback in blökkli to instead render a placeholder. If this is not possible, it should still be possible to at least delete or edit a block via the "Structure" pane.