feature (relation-widget) enable reordering for relation controls - closes 5684

[andipabst]

Summary
From issue #5684: Prepending a new value to a relation widget (which has multiple enabled) is very cumbersome. Since value re-ordering is not permitted, one must delete all existing values, add the new value followed by all pre-existing values.

This PR introduces ordering the elements in the relation widget like in the list widget.

Test plan
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simplescreenrecorder.mp4

](url)

Checklist

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[andipabst]

@erezrokah Could you please have a look? Is there anything I can improve?

[erezrokah]

@erezrokah Could you please have a look? Is there anything I can improve?

Hi @andipabst, this is 100% on me being slow to review PRs. This one is on the top of my list now that we released #5780

[erezrokah]

Great work on this @andipabst 🚀