Split off from D9054

This patch adds a "Blend To Neighbour" Operator to the pose sliding tools.

Problem
When animating you often want to nudge your current pose to the neighbouring poses slightly.

The breakdowner would override the current pose completely.

Solution
Instead of interpolating between the left and right key, this operator interpolates from the current pose to either left or right, depending if the percentage is bigger or smaller than 50%.
This graphic should explain how it works compared to the Breakdowner

There is a menu entry for the new operator. It is also registered under the ALT + SHIFT + E hotkey

Limitations and future improvements
POSE_OT_blend_to_neighbours doesn't respect an unkeyed position. In fact none of the operators do, but this hasn't been an issue until now.
The tools still only work in pose mode. It would be good for them to work in object mode as well.

Video Demonstration

Build
graphicall not working for me. so gdrive instead
https://drive.google.com/file/d/1pvXmjYjGD9g8_34FCE-1AI7YN7yQ6WHl/view?usp=sharing