Various architecture upgrades to support this.
Creating a BOARD now requires a valid PROJECT, which caused
some (mostly transparent) changes to the Python API internals.
ADDED: Project local settings file
CHANGED: Board design settings are no longer stored in PCB file
CHANGED: Net classes are no longer stored in PCB file
CHANGED: Importing board settings now reads boards, not just projects
Fixes https://gitlab.com/kicad/code/kicad/-/issues/2578
Fixes https://gitlab.com/kicad/code/kicad/-/issues/4070
Jon Evans