Complete addition of Qt::TimerId

Qt::TimerId was added in https://codereview.qt-project.org/c/qt/qtbase/+/519622 as an attempt to make a strongly typed timer ID, avoiding conflicting interpretations of invalid timers as -1 or 0.

However, the new type is used inconsistently: QTimerEvent doesn't provide a getter of that type; QObject::startTimer still returns int, while QObject::killTimer got an overload that takes a Qt::TimerId. It is not possible to use Qt::TimeId without explicitly casting from the int-ID we get from QObject::startTimer, and then it's not possible to compare it with the ID we get from QTimerEvent.

In addition, we already have a strongly-typed int representing a timer: QBasicTimer.

To make Qt::TimerId work, we need to add support for it to QObject::startTimer, QBasicTimer, and QTimerEvent. This might be an opportunity to replace storage of timer ID's and usage of QObject::start/killTimer in existing code with a QBasicTimer.