Make Incidence[Base] and Event Q_GADGETs

Summary:
This allows exposing them to QJSEngine directly. Far from all properties
covered yet, just the ones found in ical files relevant for KItinerary.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D15536