pcbnew: Add arc approximation setting to board

This places the arc approximation setting in the kicad_pcb file and uses
it for all parts of the board rendering where arcs are converted to
segments.  This allows the user to customize their speed vs. accuracy
tradeoff.  The default setting of maximum error of 0.005mm is acceptable
for small boards on moderate systems.

common/plotters/DXF_plotter.cpp

@@ -745,7 +745,6 @@ void DXF_PLOTTER::FlashPadRoundRect( const wxPoint& aPadPos, const wxSize& aSize
                                      EDA_DRAW_MODE_T aTraceMode, void* aData )
 {
     SHAPE_POLY_SET outline;
-    const int segmentToCircleCount = 64;
     TransformRoundChamferedRectToPolygon( outline, aPadPos, aSize, aOrient,
                                  aCornerRadius, 0.0, 0, GetPlotterArcHighDef() );