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Action Constraint: add Split Channels Mix choices from Copy Transforms

Practice shows that when combining actions and direct animation
it is usually best to combine location, rotation and scale
separately, which is implemented by the Split Channels modes
recently introduced in D9469 for Copy Transforms. This completes
the same set of 6 choices for the Action Constraint.

The default for new constraints is changed to the newly
added Before Original (Split Channels) mode.

The original patch is motivated by Loic Pinsard, who created
an addon that does the equivalent of this feature by splitting
the action into two, separating location and rotation+scale.

Differential Revision: https://developer.blender.org/D7547