QQuickControl::focusReasonChanged does not work properly

It's only issued when QQuickControl::setFocusReason is called which is not all cases. Normally, it will change as the focus changes through other means like a tab press or anything, which are taken care of by QQuickItemPrivate.

I'd produce a patch but it's unclear what the way forward is. I suggest we make focusReason a QQuickItem property. Alternatively, we can use one of the fancy new property objects from Qt 6 as QQuickItemPrivate is not a QObject and therefore cannot emit.