Ability to limit the template's effective content to just a small section (similar to Tapestry 4's $content$ marker)

Tapestry 4 and earlier had the "$content$" marker that would limit the content in the template. A new T5 element, <t:content>, would accomplish much the same thing. It would allow the outermost portion of a page template to be "placeholder" (for WYSIWYG), with the central part of the template marked as the <t:content>.

<t:content> would not be allowed to nest.

The Tapestry 4 $remove$ marker has already been fixed as TAP5-92.