Reinstate heirarchy constraint in ObjectSet

KrIr17requested to merge
KrIr17/inkscape:fix_objects_dialog_crash into master

Reverts changes to ObjectSet in !6371 (merged) that allowed placing both child and parent in the set.

Follow up to !6982 (merged), addresses the other part of #5454 (closed) (sorry for ninja-ing part of the issue!)

The second commit is an attempt to emulate the 'contiguous selection' behaviour that prompted the changes to ObjectSet: Children of selected groups are now highlighted (lit?) with 20% opacity. This needs some UX review due to its much broader scope.

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Also fixes #5529 (closed)

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