Gui: add new API ViewProviderDocumentObject::allowOverride()

This API is used to delcare if a view provider can be used as an
override for a give object.

Conflicts:
	src/App/DocumentObject.h

src/App/FeaturePython.h

@@ -57,7 +57,6 @@ public:
     bool onBeforeChangeLabel(std::string &newLabel);
     void onChanged(const Property* prop);
     void onDocumentRestored();
-    bool allowOverrideViewProviderName() const;
     std::string getViewProviderName();
     PyObject *getPyObject(void);
 
@@ -200,9 +199,6 @@ public:
         }
         return DocumentObject::StdReturn;
     }
-    virtual bool allowOverrideViewProviderName() const {
-        return imp->allowOverrideViewProviderName();
-    }
     virtual const char* getViewProviderNameOverride(void) const override {
         viewProviderName = imp->getViewProviderName();
         if(viewProviderName.size())