Offer the usual choices when deleting recurring to-dos

When the user deletes or drags a recurring event, KOganizer offers to act on the selected recurrence only, on it and future recurrences, or on all recurrences. KOrganizer offers the same choices when the user drags a recurring to-do, but not when deleting a recurring to-do.

This patch uses the same flow when deleting events and to-dos. Note that the choice between deleting child to-dos and making them independent is only useful when deleting all recurrences; this patch displays a second dialog instead of adding another choice to the main dialog.