If controls are reordered, relink them to mesh

[carolhmj]

Related forum issue: https://forum.babylonjs.com/t/gui-control-zindex-update-unlinks-mesh/32113

[azure-pipelines]

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[azure-pipelines]

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Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/12743/merge/index.html

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