Sort by date/time correctly in the Event List view

This patch fixes some bugs in the Event View:

 * If the Event List view contains a mix of events, to-dos, and journal
   entries, sorting by their start date/time does not order them correctly,
   because the start values for to-do and journal list items are undefined.

 * The sorting code does not consider recurring to-dos.

 * Sorting by end date/time puts the list items in the reverse of the
   correct order;  try sorting the end date/time column and the summary
   column into ascending order, and compare the up/down indicators in the
   column headers.

(Backport of !7 (closed))