PNS routing options allow trace width to be chosen by the assigned
netclass. This extends that to differential pair width/gap settings as
well.
Fixes: lp:1672126
* https://bugs.launchpad.net/kicad/+bug/1672126
- re-worked PNS_LINE_PLACER and PNS_ROUTER classes a bit, removing duplicate class members
- cleaned up Andrew's blind/buried via fixes
- fixed 'custom via width' dialog bug updating the width even when closed/cancelled
- fixed incorrect radius of drawn microvias