Nicolas Fella authored Nov 18, 2020 and Dimitris Kardarakos committed Nov 22, 2020

KCalendarCore::Calendar is the central abstraction in the upcoming
plugin system. MemoryCalendar is just one possible implementation of it.

Use the abstract type in various places and only use MemoryCalendar when
actually instantiating a calendar.

This prepares for using different kinds of calendars.