SUMMARY
When a "pure" tag group is added (i.e. a tag group that is not also used as a tag on some image/video), and then this group is added to another group, this information is lost if the compressed index.xml file format is used.

The problem is that the tag group is stored under the `<member-groups>` node in index.xml, but not as an independent node below the respective `<Category>` node. Therefore, no numeric id is assigned to the group and when it is added to another group, an ephemeral numeric id for the group is stored.
When the index.xml file is read again, kphotoalbum throws away the invalid id.

STEPS TO REPRODUCE
1. Open kphotoalbum --demo
2. Open the annotation dialog for an image (e.g. "new_wave")
3. Right-click on tag "Esbjerg"
4. Click "Tag groups | Add this tag to a new tag group..."
5. Enter "test"
6. Right-click on tag "test"
7. Click "Tag groups | USA"
8. Save database and exit without deleting the demo database

OBSERVED RESULT
Examining the `index.xml` file, member group "USA" has a member "11", but the category "Places" only has tags with ids 1 to 10.
The tag "test" is only a member group, but not a tag.
When loading this database in kphotoalbum, the tag group "test" is no longer a member of group "USA".

EXPECTED RESULT
There should be a tag node in the category "Places" with name "test" and id 11.