Display recurrence without main incidence

A disassociated occurrence of a recurring incidence can be left behind when the recurring incidence is deleted, due to bugs. This patch makes them visible, for robustness, so they can be cleaned up.

@kfunk @winterz testing would be wise; KOrganizer control flow is complicated. During start-up, AgendaView::Private::calendarIncidenceAdded is called with each disassociated and "ordinary" incidence in turn. If the disassociated occurrence is displayed before its parent is inserted, the result is the mishap I appended to MR 14.