Refactor some of the event list population logic.
Refactor some of the event list population logic.

* bin/recurrence_ui.py
  (_cmp_occurrence_by_date): New (or, really, renamed from a more
    closely scoped helper).
  (RecurrenceEventListCtrl.RegisterDatafile): Do less work, deferring
    most of the effort to...
  (RecurrenceEventListCtrl.RefreshEventList): ...this, which also employs...
  (RecurrenceEventListCtrl._AppendEventToList): ...this helper.
  (RecurrenceMainFrame.RegisterDatafile): Alert if the provided
    datafile doesn't open or parse.

cmpilato/recurrence