See https://developer.blender.org/D9499.
"View" leads to weird names like `TreeViewViewLayer` and after all they are
specific to what we call a "display mode", so "display" is more appropriate.
Also add, update and correct comments.
See https://developer.blender.org/D9499.
Also:
* Add `space_outliner/tree/common.cc` for functions shared between display
modes.
* I had to add a cast to `ListBaseWrapper` to make it work with ID lists.
* Cleanup: Remove internal `Tree` alias for `ListBase`. That was more confusing
than helpful.