Fix off-by-one with dates stored in ICAL, due to LocalZone/UTC confusion.

Summary:
Issue 1:

KDateTime(date) uses LocalZone, while we want UTC since the loading
code converts to UTC. Before this fix, setting June 1st in the GUI
would be stored as
DTSTART;TZID=Europe/Paris:20160601T000000
and then loaded back with this code
task->setStartDate(todo->dtStart().dateTime().toUTC());
which leads to 20150531 23:00:00 UTC, i.e. one day before.

Using UTC for every KDateTime(date) conversion fixes this,
and avoids 3K of VTIMEZONE information in each (!) icaldir file.

Issue 2:

The QDateTime set by the GUI has to be in UTC too, to avoid conversions.

Reviewers: ervin

Reviewed By: ervin

Differential Revision: https://phabricator.kde.org/D1836