Modifying CalendarEventData

[b-scharbau]

I currently had the problem that I wanted to modify an existing event entry (in particular change start and end time). However, I did not find an option to modify existing events in this library, so I went with the solution to delete the event from the EventController first, and then add it again, with modified values.

However, during this approach, I noticed that the event is not correctly deleted from the controller, but still remains stored in the _eventList variable, where it leads to the event supposed to be deleted being represented multiple times in the DayView after subsequent modifications (which then call remove on the controller).

[b-scharbau]

I made a PR (#37) addressing this issue, but I think the comparison algorithm might be improved upon here