Qt: Ask user to use standard port on startup if specified port is in use
Qt: Ask user to use standard port on startup if specified port is in use

If the port (specified by -port or GUI setting) is already in use when
starting the GUI client, and it's not the standard port, ask the user if
they want to listen via the standard network port instead.

Github-Pull: #7107
Rebased-From: 1f37c87d8f2fbb7113c38674d1cc65933a222ccb

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