Events with no duration (deadlines) aren't handled properly

Some calendars use events that mark a single moment rather than a span of time - "assignment due", "submission deadline", and similar. They have no duration at all: they start and end at the same instant. Bullet doesn't treat these as a proper kind of entry. In practice that means they can be skipped when a calendar syncs, and they're at risk of being removed again on a later sync, even though nothing about them has changed. These should be supported as a first-class thing - a deadline on a day, rather than an event with zero length.