Refactor ExportSpecctraFile to be usable in API

Currently there is pcbnew.ExportSpecctraDSN() but it needs a pcbnew
frame set which makes it unusable outside of the scripting window or a
plugin.

This refactors actual exporting logic into separate helper and adds an
overload to ExportSpecctraDSN(board, filename) that doesn't need pcbnew
window.

pcbnew/python/scripting/pcbnew_scripting_helpers.cpp

@@ -46,6 +46,7 @@
 #include <pcbnew_scripting_helpers.h>
 #include <project.h>
 #include <settings/settings_manager.h>
+#include <specctra.h>
 #include <project/project_local_settings.h>
 #include <wildcards_and_files_ext.h>
 #include <locale_io.h>
@@ -270,6 +271,22 @@ bool ExportSpecctraDSN( wxString& aFullFilename )
     }
 }
 
+
+bool ExportSpecctraDSN( BOARD* aBoard, wxString& aFullFilename )
+{
+    try
+    {
+        ExportBoardToSpecctraFile( aBoard, aFullFilename );
+    }
+    catch( ... )
+    {
+        return false;
+    }
+
+    return true;
+}
+
+
 bool ExportVRML( const wxString& aFullFileName, double aMMtoWRMLunit, bool aExport3DFiles,
                  bool aUseRelativePaths, const wxString& a3D_Subdir, double aXRef, double aYRef )
 {