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/DocumentObjectPy.xml

@@ -262,5 +262,11 @@ or None if the GUI is not up</UserDocu>
             </Documentation>
 			<Parameter Name="Parents" Type="List"/>
         </Attribute>
+        <Attribute Name="OldLabel" ReadOnly="true">
+            <Documentation>
+                <UserDocu>Contains the old label before change</UserDocu>
+            </Documentation>
+			<Parameter Name="OldLabel" Type="String"/>
+        </Attribute>
 	</PythonExport>
 </GenerateModel>