week-view: Minimum height of events

In week view, the widget containing the event has a height corresponding to the event duration. This is not satisfying when event duration is too small. In case of a event with no duration (start & end point at the same time), we only have a line, which is difficult to see.

Just before initializing the widget, we ensure the end point is at least 20 minutes after the beginning. Thus there is space to put one line of text in the widget.

fixes: #409