handle/key
This used to be a weird per-curve setting which would happen to get
applied/work correctly if handles were set to "auto", and was a source
of constant confusion for both old and new animators. The main effect
of this handle-type/option was really to just ensure that auto-handles
stayed horizontal, instead of tilting as the keys were moved.
This commit simply changes this from a per-curve to per
keyframe/handle setting.
900928f8bf