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.

The layer names are then used in creating the custom rig layer UI.  This is
useful for users that do not want to--or do not have the knowledge to--edit
the generated python script by hand.  It is also handy even for more advanced
users when regerating the rig over and over (which over-writes the script
and any hand-made edits).

Also misc bug fixes in some of the rig types.

the rig.

Clean-up of the IK/FK snapping code.  Should be much more maintainable now.

Also changed rig id generation.  Rig id's now consist of a random
alphanumeric string 8 characters long, with the smallest 8 digits of seconds
since the epoc (in hex) at the time of rig generation appended on the end.
This results in a 16-character string that is ludicrously unlikely to
have any collisions between rigs.  36^8 * 16^8, with the 16^8 being very well
distributed over time.  Ah... paranoia.

Also cleaned up how some of the operators work.  They weren't playing nice
with undo.