With the new automatic handle algorithm, it is possible to do a lot
of the animation via keyframes without touching the curves. It is
however necessary to change the keyframe interpolation and handle
types in certain cases. Currently the dopesheet/action editor
allows changing the types, but does not show them in any way.
To fix, add a new menu option to display this information inside the
key diamonds, simplified to 7 basic categories: Auto Clamp <>, Auto <.>,
Vector <v>, Other Bezier <X>, Linear <->, Constant <|>, Other <+>.
In addition, it is sometimes helpful to know the general direction of
change of the curve, and which keys are extremes. This commit also adds
highlighting of extremes, based purely on comparing the keyed values
with adjacent keys (not taking overshoots between keys into account).