Better structure of "elementInfo" objects.

[justinfagnani]

We have several plain JS objects that describe elements in messages, and in state held by the stage, document, and inspector elements, like the "ElementInfo" object, or an object that combines ElementInfo and the bounds.

These have grown ad-hoc and could use some reorganization and documentation, given that these are so central to editing and used in various APIs and bindings. It's likely that bounds information should be folded into elementInfo, like computed style is.