The rest shape of B-Bones is actually affected by custom handles or
the default connected parent/child mechanism. Ignoring these effects
thus leads to the edit mode shape being different from the actual
This splits the b_bone_spline_setup function that is used to compute
the correct rest and pose shape from pose channels into two parts,
and applies the data structure independent half to edit mode.
In order to access the custom handle settings in Edit Mode, they are
moved to Bone and EditBone, while the bPoseChannel fields are downgraded
in status to a cache for performance. Also, instead of flags, introduce
an enum to specify the handle operation modes, so that new ones could
be added later.