Keep track of which IDs are animated by which Action Binding. This will be necessary for display in the Action editor, where animation data that is unrelated to the active object can be shown (when "show all bindings" is on). Note: animation evaluation will not be using this cache, at least not in the near future. Potentially when we introduce animation-level constraints this will change, but that's for the future. The user cache isn't actually used in this commit. It will be used soon in !122672. Pull Request: https://projects.blender.org/blender/blender/pulls/123187 Pull Request: https://projects.blender.org/blender/blender/pulls/123187