Find trees

Support the XPath `doc(...)` function and use it to locate trees outside of the application tree. This provides access to the `clients.vwf` client container or any other global tree.

@eric79, @allisoncorey, please review. @scottnc27603, @jessmartin, fyi.

Not visibly to the application. But the drivers have to see parent/child relationships from the start of the child. Otherwise, as the child constructs, they don't have access to their data for the parent that they need to hang the child's data onto.

Calls derived from application calls, such as `node.parent` or `node.children` come to the kernel with `initializedOnly` set to `true`, which hides the connection between uninitialized children and their parents.

(model/javascript does't use this for `parent` and `children` since it maintains its own references, but it is used when `find` and `test` cross these boundaries).

Drivers don't pass this flag by default and will see the connections though. Drivers have access to this data anyway (they saw the child's `creatingNode` with the parent's ID), so hiding it would just be confusing.