Fix to method setDumpPython(). Previous implementation allowed only to SET "DumpPthon" property. Current fix allows also to UNSET "DumpPython" property using this method: if "DumpPython" is already set and this method is called - it will remove the property.
It fixes the following bug: when Dump Python is called first with checked, and after that - with unchecked "Save GUI State" checkbox -- the Python script is written with errors as IParameters has internal "DympPython" property set ON (left from the first call). Currently, it is possible to set it OFF, and it is done in SalomeApp_Application::OnDumpPython() function.
