Update qmake project to use C++17

[PMSeitzer]

See related issue #548

This change is necessary for compatibility with Qt6.

This is a major enough refactor, that it should probably be done on its own branch, and should require careful testing.

This is possibly also related to #600 and #601

[PMSeitzer]

Relatedly, the project should eventually be updated to Qt6. This will be a much more major change, as it will involve replacing QRegExp with QRegularExpression, null with QString(), and many other substitutions.
This kind of change should probably be executed very carefully, with easy ability to switch back and forth between working Qt builds.