Fix the collection of parents in TagStore's XML export.

The original version looped over only the root elements when
collecting parent-child relations. Now, the code uses the
'lookup' dictionary that contains every tag.

GTG/core/tags.py

@@ -314,7 +314,7 @@ def to_xml(self) -> Element:
 
         parent_map = {}
 
-        for tag in self.data:
+        for tag in self.lookup.values():
             for child in tag.children:
                 parent_map[child.id] = tag.id