This changes the file format. All previous copper layers that had a user
defined name are forced back to the canonical name and the user defined
name is stored as an optional quoted string in the layer definition and
only used for UI and plotting purposes. All copper object layer names
are now the canonical name for internal file use.
ADDED: Nine new user definable non-copper layers that can be optionally
added to the board layer stack.
CHANGED: All board layers can now be renamed by the user.
CHANGED: User defined layer names can now contain space characters.
Fixes https://gitlab.com/kicad/code/kicad/issues/1969
Seth Hillbrand
Jeff Young
Sylwester Kocjan
Wayne Stambaugh
Mark Roszko