Allow edits to self-intersecting polygons.

1) Intermediate states might be self-intersecting, and we shouldn't
be policing our users on what order to do things in
2) The polygon might already be self-intersecting, at which point we're
preventing the user from fixing it.

Also includes better const management for SHAPE_POLY_SET API.

Fixes: lp:1833831
* https://bugs.launchpad.net/kicad/+bug/1833831

pcbnew/plot_brditems_plotter.cpp

@@ -749,7 +749,7 @@ void BRDITEMS_PLOTTER::PlotDrawSegment( DRAWSEGMENT* aSeg )
         {
             if( !aSeg->IsPolygonFilled() )
             {
-                for( auto it = aSeg->GetPolyShape().IterateSegments( 0 ); it; it++ )
+                for( auto it = aSeg->GetPolyShape().CIterateSegments( 0 ); it; it++ )
                 {
                     auto seg = it.Get();
                     m_plotter->ThickSegment( wxPoint( seg.A ), wxPoint( seg.B ),