Improve overlap detection for zero length events in the detailed view

Where an event with a zero length duration (such as a task) is rendered, the rendering does not take into account the area required to display the event. The event box therefore overlaps other events which start soon after the zero length event, rather than creating a new column.

The attached patch gives zero length events a duration of half an hour, when calculating overlapping events. Although this cannot guarantee no overlaps, as the height of such event boxes is calculated dynamically, it does fix the majority of cases.