Use a minimum size for events in week view

These are a couple of fixes for zero length (or very short) events, especially in the week view. These would often cause lots of rendering issues due to being rendered with a zero or even negative height - or rather failing at the attempt, leaving the week view empty.

The main change of this MR is in the week-grid where it instead of using the actual range of the event a "display" range is used to calculate the allocation. This display range for events longer than a minimum (20 minutes) is their actual range, but for shorter events a minimum length is enforced. Since this is for allocation only, it still works with the range tree code that ensures there are no overlapping events but does not change any textual representation of the date.