DocumentObject: add allowDuplicateLabel/onBeforeChangeLabel()

These two APIs allow an object to control its own labeling rules.
The previous auto re-labeling for uniqueness logic is moved from
ObjectLabelObserver (resides in Application.cpp) to PropertyString.

DocumentObject::allowDuplicateLabel() is used to inform PropertyString
that the object allows duplicate label regardless of application
setting. onBeforeChangeLabel() is called before actual label change to
give object a chance to override the new label.

src/App/DocumentObjectPyImp.cpp

@@ -834,3 +834,7 @@ PyObject *DocumentObjectPy::adjustRelativeLinks(PyObject *args) {
                         PyObject_IsTrue(recursive)?&visited:nullptr)));
     }PY_CATCH
 }
+
+Py::String DocumentObjectPy::getOldLabel() const {
+    return Py::String(getDocumentObjectPtr()->getOldLabel());
+}