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.

To do to fix this issue

• In getScheduleEntryNumber, the schedule entry number is calculated. In case another schedule entry starts at the same time, add another condition which ignores the other schedule entry if it has an (alphabetically) higher activityId.
• In ScheduleEntryTest, add tests for the more specific behaviour.
• Add a test to make sure that if two schedule entries have the same activityId, they still do not have the same schedule entry number