Update rotation sliders from bone transform

The rotation sliders are updated directly from the bone transform,
allowing a finer control over the bone position.
Also, changing the bone node SetRoll method so it either simply set the
roll in rest mode or rotate the bone around itself in pose mode. This
allows to easily retrieve the Pose roll.

Finaly, changed bone widget ComputeRotationFromReferenceAxis() to be more
consistent.

See issue #13980