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Animation: Pose Slide Operator - Blend to Neighbour

Adds a new operator to the pose slider tools that blends the
current pose with the neighbouring poses in the timeline.
The operator can be called in pose mode with Shift+Alt+E
or from the "pose" menu under "In betweens/Blend to Neighbour"

Reviewed by: Sybren A. St�vel
Differential Revision: https://developer.blender.org/D9137#inline-105214
Ref: D9137