Calendar events: allow plugins to show event details

Summary:
This adds new method to the plugin API that will allow plugins to show
event details when user clicks on the event, for example by
opening KOrganizer.

Reviewers: Frameworks, mart, apol

Reviewed By: apol

Subscribers: ngraham, broulik, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D21901