Add Form.NestForm template to allow different theming for nested forms used in Object editor

Adds Form.NestedForm and uses it in editors/Object. NestedForm extends Form but uses a different template which doesn't have a form tag. This generates valid HTML, but leaves the flexibility to tweak the nested form template if needed.
I confirm it works and is nicer than my solution.
Please ask if you want details about those lines.

I still think there's some work to do on those templates, especially for the object type, as the fields titles are hidden.
[Here's a fiddle](http://jsfiddle.net/e2m4m/).