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.cpp

@@ -435,11 +435,6 @@ int FeaturePythonImp::setElementVisible(const char *element, bool visible) {
     }
 }
 
-bool FeaturePythonImp::allowOverrideViewProviderName() const
-{
-    return true;
-}
-
 std::string FeaturePythonImp::getViewProviderName()
 {
     _FC_PY_CALL_CHECK(getViewProviderName,return(std::string()));