Consistently order activities when assigning schedule entry numbers

[carlobeltrame]

As long as we don't allow manually specifying left / width of schedule entries, I would expect that the schedule entries of two activities are always ordered in the same way in the picasso. I.e. either "Fördergespräche" should always be left, or always be right (when arranged next to "Haus putzen"), not sometimes left and sometimes right.

Alternatively, provide an explicit way to specify which schedule entry should be where. But this could be nontrivial because we allow schedule entries across day boundaries.