Allow f:with tags to be nested within templates

Currently the `` 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**

``` gsp

```

**/_fields/author/books/_displayWidget.gsp**

``` gsp

```

Start with the f:with and f:display usage shown in author/show.gsp and _fields/author/books/_displayWidget.gsp, then trace how nested tags behave across the template boundary. Verify the change using an equivalent nested author/books template scenario, with f:display resolving the book title inside each g:each iteration.

Written by the indexing model from the issue text.