Gui: modernize C++: replace 'typedef' with 'using'

src/Gui/MergeDocuments.cpp

@@ -69,7 +69,7 @@ protected:
 
 private:
     std::map<std::string, std::string>& nameMap;
-    typedef std::pair<std::string, std::string> PropertyTag;
+    using PropertyTag = std::pair<std::string, std::string>;
     std::stack<PropertyTag> propertyStack;
 };
 }