[Dev] Recurrence update

[Jeffrey Harris]

Hi Folks,

I've updated the recurrence code to a point where I think it's fine for
folks to file bugs if you want to play with it.  At the moment, there's
no dialog to choose how events are modified, most modifications are
assumed to apply only to the event you change.  Frequency is the
exception to this, changing a daily rule to a weekly rule changes all
future events.

Also, changing recurrence to Once deletes all occurrences except the
first, iCal style, but it turns out this isn't what Mimi wants the
behavior to be, so don't get used to that.

Sincerely,
Jeffrey