Formation animation refactor - use animation variants instead of special move override.

The animation overrides are a little awkward. Instead, use animation variants, which allow defining different animations (and props and such) easily for different units in a formation.
This lets us clean up the special C++ code to override the walk animations.

It continues the work started by rP20631.

Original Patch By: temple

Differential Revision: https://code.wildfiregames.com/D1337