support Qt::ApplicationStates on desktop platforms too

This feature was added recently for mobile platforms. But when a laptop goes to sleep or hibernates, and then wakes up, it would be nice if a Qt application could get some kind of notification. For example hibernation is a good time to (quickly) save documents; applications that use a cloud service should be able to know that the session will need to be restarted; etc. So the idea is to try to get a notification from the QPA plugins on each platform to implement the same Qt::ApplicationState lifecycle and associated ApplicationStateChange events.

It seems that ApplicationState should be a property somewhere too, accessible from both C++ and Qt Quick. Maybe in QSessionManager?