Update standards to c++17, Qt >= 5.12, MSVC >= 2017 (#670)

Update standards to c++17, Qt >= 5.12, MSVC >= 2017 (#670)

* raise minimum standards to c++17, Qt 5.12, MSVC 2017.

* avoid std::optional::value which didn't work until macOS 10.14.

* avoid std::optional::value which didn't work until macOS 10.14.

* update runner for codeql so we have Qt >= 5.12

* try harder to get codeql working with newer Qt.

* use c++17 fallthrough attribute.

* use c++17 'if constexpr' as suggested by resharper.

* catch mkicondoc up with c++17