Having a centeral place to find a list of all library overrides should be useful for managing production scenes where library overrides are used a lot. This change adds the individually overridden properties of a data-block under the data-block itself. Just how we show modifiers, constraints or pose channels there. This way we can also expose library override operations/options better in future. There's also a filter option for the library overrides now, so they can be hidden. It is only available in the View Layer display mode though, like the other filter options. One internal change this has to do is adding more informative return values to undo pushes and the library override functions called by it. That way we can send a notifier when library overrides change for the Outliner to know when to rebuild the tree. Differential Revision: https://developer.blender.org/D7631 Reviewed by: Andy Goralczyk, Bastien Montagne, William Reynish