Use QDate as key in mIncidencesForDate in MemoryCalendar

Summary:
Avoids unnecessary conversions from QDate to QString, since QDate does
have a qHash() function in Qt5. Also might save some bytes per each event
as QDate is more lightweight than storing a QString representation of a
date.

Reviewers: vkrause

Reviewed By: vkrause

Subscribers: ognarb, KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D25166