[Html-widget] Behavior of Callback

[Carl Franks]

Ok, I've committed this so people can test it.
Here's the Changelog:

        - New render_errors() method on constraints, accepts a list of
          element names for which errors should be displayed by $r->as_xml.
        - When an Equal constraint fails, the first named elements' value is
          no longer considered valid.
        - All elements associated with a failing Equal constraint receive an
          error. Use render_errors() to stop the error being shown on all
          elements.
        - New HTML::Widget::Error method no_render(), used internally by
          $constraint->render_errors().

Note that I've changed display_errors() to render_errors(), as it
feels more in-line with the jargon used in the documentation.

Carl