Add WooWoo HTML preview support for the only metablock item

[davidstraka2]

Add WooWoo HTML preview support for the only metablock item.

This is sometimes added to block-type content to restrict its output to only certain formats (such as pdf, html, etc.). The preview should obviously show the output regardless, but it should somehow indicate that the only item has been set.

One simple way to achieve this would be adding an "info box" around the tagged content, similar to the way document objects are displayed. Since this metablock item can be added to basically any block-type content, instead of creating some "mixin" or a function to be used in every renderer, a rendering "middleware" system could be developed.