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.

Added. I didn't change the spacing since it's consistent with the other functions in this object and the rest of the file, and I don't want to do a wholesale change here.