Support "override_previous_animation" in Animation controllers

[Cubouse]

Detailed description of your suggestion

So that an animation bones' keyframes override the ones applied before.

I'm not sure whether that should also work when we just play additive animations manually, directly in the Animations tab (using the order the animations are in); it might prove annoying to move them up and down to preview the result, and then having to put them back in an another order to stay organized.