MDL-58087 core_calendar: Don't pass non action events through our plumbing

MDL-58087 core_calendar: Don't pass non action events through our plumbing

The event factory should simply instantiate an event for non action
events and pass only action events through the plumbing.

This way modules implementing the callbacks can be sure that what
is passed in to the callback is actually supposed to be an action event.

Part of MDL-55611 epic.