Add QChronoTimer

We can't port QTimer to std::chrono (or even qint64) timeouts before Qt 7 (QTBUG-67383), but we could provide, as a stop-gap, a new class QChronoTimer that then becomes Q7Timer.

Now QTimer::start(int msec) accepts only int values which corresponds to ~ 24 days as maximum time interval. Please change interval type in QTimer to qint64 to allow more easily create schedulers with big time intervals - months, etc.

QTimer::start(std::chrono::milliseconds msec) does not work for this as value converted to int inside it, which is wrong I think.