Explicitly import Qt 5.7's QtQuick for Connections.enabled

Summary:
It was introduced in that version. Fixes 'qrc:/DateTimeEdit.qml:60
"Connections.enabled" is not available in QtQuick 2.5.' when trying
to start itinerary, preventing its main window from appearing.

Test Plan: itinerary's main window comes up

Reviewers: vkrause

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D17184