Improve handling of root constraints in conjunction with grouping (#1552).

Details:
- Editor improvements for feature model root constraints.
- Added repositioning of feature model root constraints on constraint group assignment changes.
- Improved the the root constraints editor according to the review comments.
- Updated CHANGELOG.md.

Author: Torsten Goerg

CHANGELOG.md

@@ -19,6 +19,8 @@ The project does _not_ follow Semantic Versioning and the changes are documented
 
 - Variability: Remove deprecated concepts `VariabilityModelChunk_old`, and some related concepts.
 - Typesystem: All usages of colon casts were transformed to the 'as' cast for null safety. From now on no colon casts are allowed in the typesystem, also read this issue for more info: https://github.com/IETS3/iets3.opensource/issues/688
+- Variability: The grouping of root constraints has been improved. New root constraints will be added next to the other constraints in the respective groups. This helps to avoid merge conflicts.
+
 
 ## November 2025