Qt6: restore QRegExp port, remove Qt6Core5Compat dependency

[matterhorn103]

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[ghutchis]

This is fine, but I think we should just tweak MoleQueue to use QRegularExpression or migrate the RPC client code to avogadroapp. I would like to keep the ability to use RPC -- it's very handy to have programs tell Avogadro "hey can you open and visualize this file" 😄