calendar: Always allow TODOs to have both a start and end date

While it might sound silly, this is a feature many TODO apps have.

I personally use it to set reminders for things to do that I cannot start on until a certain time (i.e. assignments only released at a specific time) - and while I could edit such TODOs when created from another app before, I wasn't able to create them from Merkuro

This also comes with a slight fix to the IncidenceWrapper class, to avoid it messing with incidence end times too much.