QSystemDeviceInfo should report on the system shutdown state

QSystemDeviceInfo should report on the current shutdown state reason and corresponding signal.

with these enumerator values:

Shutdown - Normal shutdown
ThermalStateFatal -Shutdown due to thermal state
BatteryStateEmpty - Shutdown due battery empty within few seconds
SaveData - Save data
RebootDeniedUSB - Reboot denied because USB is connected in mass storage mode
ShutdownDeniedUSB - Shutdown denied because USB is connected in mass storage mode