[development] Forms APIing our theme system

[Nedjo Rogers]

Changing the way we handle theme() calls is great, but what about the data 
we're passing them? Currently it's a mish-mash of various numbers of 
arguments, data types, etc.

Here's another place the Forms API shines. We pass a well defined, 
structured data set. Clean separation of content and presentation.

While we're making changes, we could consider a full move to a Forms API 
approach for all data passed to theming. Likely doing so would greatly 
simplify the theming system.

* 'node', 'block', etc. element types
* generate the whole page as a nested array data structure and then pass 
once to theming

(Then if someone wants e.g. to use XSLT to generate a full SVG environment, 
easy. Serialize the data array as XML, pass to the XSL, done.)