Fix TQTaggable when global overwite activated

Release notes

Actually fix the unexpected behaviour of TQTaggable. When fGlobalOverwrite=true and a tag list is updated, all entries of the old list are deleted and the new one is written.

Example: Update a list a,b,c to d,e. Previous result: d,e,c. Now: d,e.

Details

This fix should have been included in !345 (merged).