Allow custom layouts in surveys

I needed to be able to use more flexible survey layouts than what the module allows by default, so I patched my install to add a "layout" textarea to the "fields" tab that lets you specify markup for displaying form fields. You plug form fields into the markup by using id-based placeholders (e.g. {22} for form field 22). The patch includes the following fixes:

- Add field id display for reference to the first column of field display.
- Add layout textarea to field display
- Add code to handle saving layout
- Add field to database to contain layout
- Add new form callback that breaks the layout into chunks on {\d*} markers and adds chunks as #prefix and #suffix attributes as appropriate to modify the form display
- Add new _submit function to accompany form callback that just forwards results to the original _submit function.
- Make survey_view call the new callback if $node->layout isn't empty.

I think that's it. Seems to work pretty well on my install, and I thought I'd share it for the benefit of others.