Allow deletion tracking to be disabled (KF5)
Currently, deletion tracking (which stores all Incidences deleted from the calendar) cannot be disabled except in classes derived from Calendar, since its API is protected, not public.
This effectively results in a memory leak when a large number of Incidences are created and deleted. This has actually resulted in a KAlarm crash - see https://bugs.kde.org/show_bug.cgi?id=468640.
This MR changes the deletion tracking methods to public. This will then allow KAlarm to be changed to disable deletion tracking, which should fix the bug.
Note that deletion tracking has already been removed for KF6, so this patch only applies to KF5.