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
Save/restore default netclass.
Read layers back in with correct syntax ('.' between keyword and layer name).
Write layers to project file.
Read/write layer enablement bits.
Don't set solder paste margin min to 0 (most of them are negative).
Fixes: lp:1811990
* https://bugs.launchpad.net/kicad/+bug/1811990