Fix the collection of parents in TagStore's XML export

[gycsaba96]

The original XML export for tags looped over only the root elements when collecting parent-child relations. Now, the code uses the lookup dictionary containing every tag.

As far as I know, there is no open issue for this bug.

Detailed steps to reproduce the bug:

1. Create a deep tag structure with four levels.

   - a
       - b
           - c
               - d
   

2. Close the program.

3. You can already see the faulty export in the gtg_data.xml file.

   <taglist>
    <tag id="e21eac43-35d2-480e-8818-5b86dc48b20d" name="a" nonactionable="False"/>
    <tag id="b5a629bc-b146-42b8-a6d0-26a651285d7b" name="b" nonactionable="False" parent="e21eac43-35d2-480e-8818-5b86dc48b20d"/>
    <tag id="cece686c-607d-4388-9bd1-11560d8b32ac" name="c" nonactionable="False"/>
    <tag id="76422fdf-09c0-4902-bbf3-609fe446c08f" name="d" nonactionable="False"/>
   </taglist>
   

4. If you reopen the program, you can see the tag tree has only two levels instead of four.

   - a
       - b
   - c
   - d
   

[diegogangl]

Works as advertised, thanks!