Blender Git Loki

Animation: Pose sliding tools improvements

Improve the "In Betweens" tools:
- Push Pose from Rest Pose
- Relax Pose to Rest Pose
- Push Pose from Breakdown
- Relax Pose to Breakdown
- Pose Breakdowner

These all now use the same new sliding tool:
- Actual visual indication of the blending/pushing percentage applied.
- Mouse wrapping to allow for extrapolation without having to worry
about the initial placement of the mouse. This also means these tools
are actually usable when chosen from the menu.
- Precision mode by holding {key Shift}.
- Snapping to 10% increments by holding {key Ctrl}.
- Overshoot protection; by default the tool doesn't allow overshoot
(lower than 0% or higher than 100%), and it can be enabled by pressing
{key E}.
- Bones are hidden while sliding, so the pose itself can be seen more
clearly. This can be toggled by pressing {key H} while using the tool.

Reviewed By: #animation_rigging, zeddb, sybren, #user_interface, brecht, Severin, looch

Maniphest Tasks: T81836

Differential Revision: https://developer.blender.org/D9054