This version makes use of lots of things learned going down the
other rat hole. Avoiding the wxComboFocusHandler is key, as well
as specifying using the AltPopupWindow.
The key handler is still tricky with respect to those platforms
that use native controls, but the starting-key strategy is similar
to the one used with wxGrid text editors.
Also fixes a bug where nets wouldn't get changed because the
pads weren't changed and so the pad nets would propagate and
wipe out the changed nets on the tracks.
Also includes warning dialogs that pad nets will be changed if the
track nets are.
Fixes: lp:1779854
* https://bugs.launchpad.net/kicad/+bug/1779854
