[BACK_INCOMPAT] Split `fabric.Object.centerTransform` and `fabric.Canvas.centerTransform` into the properties `centeredScaling` and `centeredRotation`
centeredScaling: Define if object scaling should be centered (true). Default = false.
centeredRotation: Define if object rotates around center (true) or around origin (originX/originY) point (false).  Default = false.
Pressed altKey negates the behavior.
Add missing doc additions of previous commit from kangax
Merge pull request #872 from Kienz/patch-1

[BACK_INCOMPAT] Split `centerTransform` into the properties `centeredScaling` and `centeredRotation` - Fixes #782