Allow f:with tags to be nested within templates

[tcrossland]

Currently the <f:with> tag can be nested but only within a single pageScope. It would be useful to allow nesting to work within templates.

For example:

/author/show.gsp

<f:with bean="authorInstance">
  <f:display property="books"/>
  <f:display property="name"/>
</f:with>

/_fields/author/books/_displayWidget.gsp

<ul>
  <g:each in="${value}" var="book">
    <f:with bean="${book}">
      <li><f:display property="title"/></li>
    </f:with>
  </g:each>
</li>