App: add PropertyXLinkList

PropertyXLinkList is derived from PropertyXLinkSubList, which is
modified to provide APIs compatible to PropertyLinkList.

The only difference between the two class is in their getPyObject() to
make PropertyXLinkList output similar to PropertyLinkList in Python. In
C++, both class can accept mix of objects link with or without subname,
depending on which set of APIs are used.

src/App/Application.cpp

@@ -1702,6 +1702,7 @@ void Application::initTypes(void)
     App ::PropertyXLink             ::init();
     App ::PropertyXLinkSub          ::init();
     App ::PropertyXLinkSubList      ::init();
+    App ::PropertyXLinkList         ::init();
     App ::PropertyXLinkContainer    ::init();
     App ::PropertyMatrix            ::init();
     App ::PropertyVector            ::init();