add support for Qt6 / C++17 #1198

Job	Run time
canceller	2s
tests-Unix-CLI (ubuntu-20.04, 7, 3.8)	11m 36s
tests-Unix-CLI (ubuntu-20.04, 11, 3.9)	11m 42s
tests-Unix-GUI (ubuntu-20.04, 5.9.5, 7)	3m 26s
tests-Windows-CLI (mingw64, x86_64, 3.8)	23m 7s
tests-Unix-CLI (macos-12, 3.8)	21m 12s
tests-Unix-GUI (ubuntu-20.04, 5.15.2, 11)	5m 30s
tests-Windows-GUI (mingw64, x86_64)	12m 13s
tests-Windows-CLI (ucrt64, ucrt-x86_64, 3.8)	21m 0s
tests-Unix-CLI (macos-13, 3.9)	16m 39s
tests-Unix-GUI (ubuntu-22.04, 5.15.2, 12)	6m 27s
tests-Windows-GUI (ucrt64, ucrt-x86_64)	12m 8s
tests-Unix-GUI (macos-12, 5.9.5)	5m 48s
tests-Unix-GUI (macos-13, 5.15.2)	7m 6s
tests-windows-latest-pacman (mingw64, x86_64)	1m 37s
tests-windows-latest-pacman (ucrt64, ucrt-x86_64)	1m 36s
	2h 41m 9s

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