libdeng2|BinaryTree: Numerous improvements to the interface of de::BinaryTree<>

- Methods for returning references to linked subtree elements.
- Throws de::Error exceptions where subtree elements are missing.
- Added a plethora of new convenient accessor/inspection methods.
- Improved apidoc.
Refactor: Collate the BspBuilder's HEdges in collateBSP

It is now unnecessary to split this job into a secondary traversal
as the builder now constructs two trees in parallel (one for the
purposes of guiding the build process and the other containing the
work-in-progress BSP tree elements).
Refactor: Applied the pimpl macro in BspBuilder; cleanup