[TD]Use PropertyXLinkList for external Sources

src/Mod/TechDraw/App/DrawViewPart.h

@@ -93,6 +93,7 @@ public:
     virtual ~DrawViewPart();
 
     App::PropertyLinkList     Source;
+    App::PropertyXLinkList    XSource;
     App::PropertyVector       Direction;  //TODO: Rename to YAxisDirection or whatever this actually is  (ProjectionDirection)
     App::PropertyVector       XDirection;
     App::PropertyBool         Perspective;
@@ -202,6 +203,9 @@ public:
     void removeAllReferencesFromGeom();
     void resetReferenceVerts();
 
+    std::vector<App::DocumentObject*> getAllSources(void) const;
+
+
 protected:
     bool checkXDirection(void) const;