Custom handle settings actually affect the B-Bone rest shape,
so they should be changed in Edit mode rather than Pose mode.
This is necessary to be able to display the correct rest shape
of the bone in Edit Mode.
Also, instead of flags, introduce an enum to specify the handle
operation modes, so that new ones could be added later.
Differential Revision: https://developer.blender.org/D3588
61a24c799b