Add support for std::chrono::duration from QML

Some Qt APIs suffer from weakly typed durations, and it can be hard to see the unit of durations from code.

This could be improved by supporting properties with std::chrono durations, such as std::chrono::milliseconds, as QML properties.

See https://codereview.qt-project.org/c/qt/qtmultimedia/+/632065/ for an example where a networkTimeoutMs property would be better expressed with std::chrono::milliseconds than with ints.