Hmm, I seem to be more productive than usual tonight. I decided to go ahead and lay down some of the groundwork for decoupling SPObjects and ids.

To begin with, I've replaced sp_document_lookup_id with two methods:

* SPDocument::getObjectById()

* SPDocument::getObjectByRepr()

They do pretty much what you expect; I've already rewritten all the code in the codebase to getObjectById(), or getObjectByRepr() where it was simple and obvious to do so.

(getObjectById() should stay at least for some things like Inkscape::URIReference)

As before, I've left sp_document_lookup_id as a deprecated wrapper, but it will go away soon.

-mental