Pcbnew: added: SVG plotter. Need refinements, but works.

Mainly to plot drill maps, but can be used to plot boards, for documentation.
The print svg still exists, but the plot SVG has more options (mirroring, holes in pads),
however  print svg allows color print, and full board printing, and plot does not.

common/common_plot_functions.cpp

@@ -36,6 +36,9 @@ wxString GetDefaultPlotExtension( PlotFormat aFormat )
     case PLOT_FORMAT_GERBER:
         return GERBER_PLOTTER::GetDefaultFileExtension();
 
+    case PLOT_FORMAT_SVG:
+        return SVG_PLOTTER::GetDefaultFileExtension();
+
     default:
         wxASSERT( false );
         return wxEmptyString;