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.

source/blender/editors/include/UI_interface.h

@@ -3286,7 +3286,7 @@ char *UI_tree_view_item_drop_tooltip(const uiTreeViewItemHandle *item, const str
  * \return True if the drop was handled by the tree-view item.
  */
 bool UI_tree_view_item_drop_handle(struct bContext *C,
-                                   uiTreeViewItemHandle *item_,
+                                   const uiTreeViewItemHandle *item_,
                                    const struct ListBase *drags);
 /**
  * Can \a item_handle be renamed right now? Not that this isn't just a mere wrapper around