Blender Git Loki

Pose library: Use scrollwheel to set the blend factor

This is not the final UI, but gives a way to at least set it to some value.

Cleanup: remove unused code and rename variables/fields

No functional changes.

Add operator `POSELIB_OT_blend_pose` for interactive pose blending

The operator doesn't blend yet, but it does allow temporarily applying the
pose, switching between the original and the given pose with Tab, applying
the pose with Enter/LeftClick, and cancelling with Esc/RightClick.