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.

1 files changed, 1 insertions, 1 deletions

diff --git a/source/blender/editors/space_outliner/tree/tree_element_gpencil_layer.hh b/source/blender/editors/space_outliner/tree/tree_element_gpencil_layer.hh
index dd18dd42344..da57ef63f1f 100644
--- a/source/blender/editors/space_outliner/tree/tree_element_gpencil_layer.hh
+++ b/source/blender/editors/space_outliner/tree/tree_element_gpencil_layer.hh
@@ -26,7 +26,7 @@ struct bGPDlayer;
 
 namespace blender::ed::outliner {
 
-class TreeElementGPencilLayer : public AbstractTreeElement {
+class TreeElementGPencilLayer final : public AbstractTreeElement {
  public:
   TreeElementGPencilLayer(TreeElement &legacy_te, bGPDlayer &gplayer);
 };