Form nesting and compositing

[stickgrinder]

As of 2.0 there is no way to "import" a form into another one. Say I have an "Address" form and a "complete your order" one. While completing an order the user should specify both billing and shipping addresses. I could then just import/embed "Address" form twice, reusing code.

Some note on this:

• Add a parameter to import form as items only, wrapped within a fieldset and/or within a section (see issue Possibility to nest fieldsets #3)
• Add a parameter to specify field naming in relation to holding form