Continuation of work in 2e221de4ce, 249e4df110 and 3a907e7425.
Adds new tree-element classes for the scene-ID, scene collections, scene
objects, and the view layers base.
There is some more temporary stuff in here, which can be removed once we're
further along with the porting. Noted that in comments.
See https://developer.blender.org/D9499.
It's odd to include a C++ header (".hh") in C code, we should avoid that. All
of the Outliner code should be moved to C++, I don't expect this C header to
stay for long.
