Significant improvement in fetch time for item clearances. On large
boards with lots of nets, maybe 10% faster zone fills and about
2x speedup on track-to-track DRC.
Fixes a few commit issues based on the std:: rework. Add an option to
delete tracks fully in pads and makes merge tracks smarter
Fixes: lp:1787190
* https://bugs.launchpad.net/kicad/+bug/1787190
... and GetMsgPanelInfo.
Step 4 in the g_UserUnit eradication effort.
Also removes a couple of conversion routines that were close
enough to extinction.
(cherry picked from commit c75da51)