Enable zooming in scheduler GUI

Ideally separate in both directions(?), but ctrl + scroll would do just fine (sorted out in !146 (merged)).

added Scheduling GUI label

mentioned in merge request !146 (merged)

changed the description

changed milestone to %Summer 2024

Would still be useful to have separate zoom in x- and y-direction. Possible a "zoom to fit" that sets the zoom so that the whole space is used, not just in the limiting direction.