Glen Ditchfield authored Jun 09, 2020 and Allen Winter committed Jun 10, 2020

`Todo::iconName()` compares a "recurrenceId" to the due date of the current
occurrence of the todo to decide whether the specified occurrence has been
completed.
 * To-dos do not have to have a due date.  In that case the comparison
   fails, and `"view-calendar-tasks"` is always returned.
 * RFC 2445/RFC 5545 say that a recurrence ID is the start date of an
   occurrence, and KOrganizer's Agenda view passes start dates to
  `iconName()`.  In that case `iconName()` incorrectly returns
  `"task-complete"` for the first occurrence of a never-completed to-do.

Comparing the recurrence ID to the start date of the current occurrence
fixes the problems.