GTK uses a level of cooperative multitasking when dealing with
secondary event loops. This gives the wm enough leeway to process the
redraw before returning to the secondary loop
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
Jeff Young
Seth Hillbrand