OPC-UA: Update CTN "ChangesToSave" status when variable selection change.

- fixes changes not being saved when no other changes happening to configuration
- added CTNMarkModified to ConfigTreeNode in order to generalize

<?xml version='1.0' encoding='utf-8'?>
<BeremizRoot xmlns:xsd="http://www.w3.org/2001/XMLSchema" URI_location="LOCAL://">
  <TargetType/>
</BeremizRoot>