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, 9 insertions, 0 deletions

diff --git a/source/blender/editors/space_outliner/tree/tree_element.hh b/source/blender/editors/space_outliner/tree/tree_element.hh
index 8a1ebb51eae..3e61dd25898 100644
--- a/source/blender/editors/space_outliner/tree/tree_element.hh
+++ b/source/blender/editors/space_outliner/tree/tree_element.hh
@@ -48,6 +48,15 @@ class AbstractTreeElement {
   virtual void expand(SpaceOutliner &) const
   {
   }
+
+  /**
+   * Just while transitioning to the new tree-element design: Some types are only partially ported,
+   * and the expanding isn't done yet.
+   */
+  virtual bool isExpandValid() const
+  {
+    return true;
+  }
 };
 
 }  // namespace blender::ed::outliner