Add pref for new Dxf Exporter

src/Mod/Draft/Resources/ui/preferences-dxf.ui

@@ -6,8 +6,8 @@
    <rect>
     <x>0</x>
     <y>0</y>
-    <width>506</width>
-    <height>647</height>
+    <width>649</width>
+    <height>773</height>
    </rect>
   </property>
   <property name="windowTitle">
@@ -52,6 +52,19 @@
      </property>
     </widget>
    </item>
+   <item>
+    <widget class="Gui::PrefCheckBox" name="checkBox_7">
+     <property name="text">
+      <string>Use legacy python exporter</string>
+     </property>
+     <property name="prefEntry" stdset="0">
+      <cstring>dxfUseLegacyExporter</cstring>
+     </property>
+     <property name="prefPath" stdset="0">
+      <cstring>Mod/Draft</cstring>
+     </property>
+    </widget>
+   </item>
    <item>
     <widget class="QGroupBox" name="groupBox_2">
      <property name="title">

src/Mod/Draft/importDXF.py

@@ -1940,8 +1940,14 @@ def getStrGroup(ob):
 def export(objectslist,filename,nospline=False,lwPoly=False):
 
     "called when freecad exports a file. If nospline=True, bsplines are exported as straight segs. lwPoly=True is for OpenSCAD DXF"
-
     readPreferences()
+    if not dxfUseLegacyExporter:
+        import Import
+        version = 14
+        if nospline:
+            version = 12
+        Import.writeDXFObject(objectslist,filename,version,lwPoly)
+        return
     getDXFlibs()
     if dxfLibrary:
         global exportList
@@ -2307,6 +2313,7 @@ def readPreferences():
     global dxfMakeBlocks, dxfJoin, dxfRenderPolylineWidth, dxfImportTexts, dxfImportLayouts
     global dxfImportPoints, dxfImportHatches, dxfUseStandardSize, dxfGetColors, dxfUseDraftVisGroups
     global dxfFillMode, dxfBrightBackground, dxfDefaultColor, dxfUseLegacyImporter, dxfExportBlocks, dxfScaling
+    global dxfUseLegacyExporter
     dxfCreatePart = p.GetBool("dxfCreatePart",True)
     dxfCreateDraft = p.GetBool("dxfCreateDraft",False)
     dxfCreateSketch = p.GetBool("dxfCreateSketch",False)
@@ -2324,6 +2331,7 @@ def readPreferences():
     dxfUseDraftVisGroups = p.GetBool("dxfUseDraftVisGroups",False)
     dxfFillMode = p.GetBool("fillmode",True)
     dxfUseLegacyImporter = p.GetBool("dxfUseLegacyImporter",False)
+    dxfUseLegacyExporter = p.GetBool("dxfUseLegacyExporter",False)
     dxfBrightBackground = isBrightBackground()
     dxfDefaultColor = getColor()
     dxfExportBlocks = p.GetBool("dxfExportBlocks",True)