Blender Git Loki

Cleanup: Pass anim-data directly to Outliner anim-data tree element constructor

Rather than letting the `TreeElementAnimData` constructor take an ID from which
we get the animation-data based on an assumption on how it's stored, let the
constructor take the animation-data directly. That way we further centralize
the assumptions on the data passed to the element creation to
`tree_element_create()`.
The following commit will add a comment explaining the plan to entirely get rid
of those assumptions in the future.