Blender Git Loki

Cleanup: Various cleanups to the tree-view API

* Correct URL for documentation (was changed recently).
* Add comments.
* Reevaluate and update which functions are public, protected or
private.
* Reorder functions and classes to be more logical and readable.
* Add helper class for the public item API so individual functions it
uses can be made protected/private (the helper class is a friend).
Also allows splitting API implementation from the C-API.
* Move internal layout builder helper class to the source file, out of
the header.
* More consistent naming.
* Add alias for item-container, so it's more clear how it can be used.
* Use const.
* Remove unnecessary forward declaration.