Add synchronization mechanism for entering sleep mode.

When the APT status is APP_PREPARE_SLEEPMODE, the application main thread should call aptSignalReadyForSleep() to signal that it is ready to enter sleep mode, and then call aptWaitStatusEvent() as usual.

Example code:

APP_STATUS status;
while ((status = aptGetStatus()) != APP_EXITING)
{
	if(status==APP_RUNNING)
	{
		// application logic here
	}
	else if(status == APP_SUSPENDING)
	{
		aptReturnToMenu();
	}
	else if(status == APP_PREPARE_SLEEPMODE)
	{
		aptSignalReadyForSleep();
		aptWaitStatusEvent();
	}
}

This maybe isn't the proper/recommended way to do sleep mode, but I tested it multiple times and it always worked reliably.

(note: maybe the sample code above will not work if GPU drawing is done in a separate thread, haven't tested that)