Add new element, t:remove, that removes a portion of a template

Currently, any XML comments in a Tapestry template are dutifully echoed down to the client.

It would be nice to have a <t:comment> element, as a parser directive, that would support server-side comments in the template.

Such an element would not convert into a template token; it would simply drop out.

This component would fulfill the role of jwcid="$remove$" from Tapestry 4.