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
wxWidgets has several bugs that result in cell editors being closed
right after they're opened. There are two wxWidgets hacks to
partially address this: the SetInSetFocus() hack, and a slow-click
hack. We used to try and work-around these bugs ourselves for
single-click access, but this changelist moves those over to
wxWidget's slow-click hack.
Fixes: lp:1817965
* https://bugs.launchpad.net/kicad/+bug/1817965
Jeff Young
Thomas Pointhuber