Sketcher: allow referencing everything; add testing for circular references

src/Mod/Sketcher/App/SketchObject.cpp

@@ -1707,7 +1707,14 @@ bool SketchObject::isExternalAllowed(App::Document *pDoc, App::DocumentObject *p
     // Externals outside of the Document are NOT allowed
     if (this->getDocument() != pDoc)
         return false;    
+    try {
+        return this->testIfLinkDAGCompatible(pObj);
+    } catch (Base::Exception &e) {
+        Base::Console().Warning("Probably, there is a circular reference in the document. Error: %s\n", e.what());
+        return true; //prohibiting this reference won't remove the problem anyway...
+    }
 
+    /*
     // Note: Checking for the body of the support doesn't work when the support are the three base planes
     App::DocumentObject *support = this->Support.getValue();
     Part::BodyBase* body = Part::BodyBase::findBodyOf(this);
@@ -1723,6 +1730,7 @@ bool SketchObject::isExternalAllowed(App::Document *pDoc, App::DocumentObject *p
     }
 
     return true;
+    */
 }
 
 int SketchObject::addSymmetric(const std::vector<int> &geoIdList, int refGeoId, Sketcher::PointPos refPosId/*=Sketcher::none*/)