Sub-fields as position: titlebar in admin panel UI

[bratvanov]

When building complex layout editors, I aim to keep key content/data fields visible in the admin panel while "hiding" secondary styling fields. This prevents the admin UI from becoming overly cluttered, especially with nested block or array fields.

To address this, I propose a new position: titlebar admin field option. This would apply to sub-fields of titlebar-enabled fields (e.g., block, array, collapsible), making titlebar sub-fields accessible via a modal, triggered by a new icon in the parent field's titlebar (similar to the UI for editing inline rich text blocks).