Diagonally stack / stagger overlapping time events in week view's timetable

A bit similar to bug #732820 I had filed in California, except that the issue here is that GNOME Calendar only puts events side-by-side inside a day's column in weekview, instead of partially overlapping them.

If you have foo from 10h00 to 11h00 and bar from 10h30 to 11h30, you will see this in week view:

---------------
| foo         |
|             |-------------|
--------------| bar         |
              |             |
              ---------------

Instead of:

---------------
| foo         |
|      |-------------|
|------| bar         |
       |             |
       ---------------

...which uses less of the scarce horizontal space inside a day's column.

Link to original bug (#777422)