Add support for Calendar Solspace Events

[samuelreichor]

Hi @sebastian-lenz would it be very time-consuming if you add the possibility to make the events of the calendar plugin linkable. I can currently select and save them without errors, but no data arrives in the frontend. :(

Maybe the problem is that this calendar plugin does not create urls for the individual events. At least the slugs it generates are not unique. If this is the case, I would remove the option to select events completely, as otherwise it leads to confusion, I think.