Add notifications about next lesson after previous lesson

[p0358]

So that you know what your next lesson is right after one ends. Can be without sound and vibration, but it needs to be sent to wearable devices as well (smart watches and bands), so it probably can't be persistent notification. With offset option, as school clocks are not always well-synchronized.
It'd be also cool if one could set class (sala lekcyjna) for the lesson in timetable, so that it would be shown in the notification as well.