Major restructuring: the Animation class is now an abstract interface,

Major restructuring: the Animation class is now an abstract interface,
and specific animation types are derived from it.  This change makes
the code much easier to read, maintain, and extend.

Added a 'translate' animation type for a scaled, 3D linear translation
(such as a sliding throttle knob).

Renamed the 'offset' property to 'offset-m' or 'offset-deg' as
appropriate; ditto for 'min' and 'max' properties.