Allow adding events to the planner as tasks

Now you can drag a task to the calendar to create an event and block time.

We also need to do this the other way: adding a calendar event to the planner. This way you can have a single view with all the things you need to do today, including events. Also, some events can be completed before the time allocated to them is finalized, so you might want to mark them as done before they reach their end time.