Plus a few styling enhancements.

[[Split portion of a mixed commit.]]

- remove/comment unused variables
- remove unused imports
- fixed some bugs using incorrect variables

This is the last piece that should allow some pretty extensive custom rigging
in the metarig, if advanced users choose to do so.

This allows advanced users to do certain limited custom rigging work
in the metarig, and have it transfer to the generated rig.
Custom properties are also copied when using the copy_bone function
in utils.
TODO: transfer drivers from the metarig as well.

This rig replaces the old spine rig, and has a super-set of the old spine's
features.  The main new features are:
1. A separate over-all control for translation.  Due to the pivot slide
   feature of the spine, it didn't really make sense to have one of the
   spine bones be the control for translation, so I broke that out
   into a separate control.  This control also acts as a root of the
   spine in general, including for scaling and rotation.  If you
   want to grab the entire spine as one unit, this is how to do it.
2. The spine can now have more than two control bones.  The rigger
   can specify an arbitrary number of the spine bones to be
   turned into controls upon rig generation.
   Controls that are not at the end points of the spine
   are optionally (via an animatable switch) auto-rotated by the
   the end point controls, so animators can ignore them when they
   do not require that level of control.

setup (not yet added to the default human metarig).

update_tag() tags for depsgraph update only, update() executes normal recalculation and creates edge data.