korganizer CalendarView: port KCalCore connects to PMF or lambda syntax

Summary:
The lambda is needed because the slot signature is
void selectDates(const KCalCore::DateList &, const QDate &preferredMonth = QDate());
and the signal only emits one DateList argument.

Test Plan: It compiles.

Reviewers: vkrause

Reviewed By: vkrause

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D22839