Blender Git Loki

Outliner: Barebones to port IDs to new Outliner tree-element code design

Continuation of work in 2e221de4ceee and 249e4df110e0 .
This prepares things so we can start porting the individual ID types to
the new code design. I already added some code for library IDs, because
they need some special handling during construction, which I didn't want
to break.

The `AbstractTreeElement::isExpandValid()` check can be removed once
types were ported and can be assumed to have a proper `expand()`
implemenation.

Also makes `TreeElementGPencilLayer` `final` which I forgot in
e0442a955bad.