Use ISO date/time in csv export

CSV is a machine readable format, but localized date/times complicate parsing. I was trying to write a python3 script to parse the CSV output but since python's datetime doesn't parse unknown date formats this would be impossible without using a 3rd party python library.

Hmmm, I could set LC_TIME=C to have a consistent date format to parse, but future users might benefit from ISO dates.

I've no idea if this patch will break other software using konsolekalendar, as they might be using the LC_TIME=C too.

$ LC_TIME=C konsolekalendar --export-type csv 2>/dev/null
"Wednesday, July 29, 2020","","Wednesday, July 29, 2020","","test allday","Default location","","fea4c06b-f818-47b1-bbfe-0dc3eae9dbfb"
"Wednesday, July 29, 2020","8:00:00 AM EDT","Wednesday, July 29, 2020","9:00:00 AM EDT","test time","Default location","description","f605a223-b27f-4049-95bf-20b2deea67d4"

• https://github.com/KDE/akonadi-calendar-tools/blame/master/konsolekalendar/konsolekalendarexports.cpp#L223
• https://github.com/KDE/kcalendarcore/blob/master/src/event.h
• https://doc.qt.io/qt-5/qdate.html#toString-1
• https://doc.qt.io/qt-5/qtime.html#toString-1

If Merged TODO:

• https://docs.kde.org/trunk5/en/pim/konsolekalendar/formats.html

Before:

$ LC_TIME=fr_FR.UTF-8 konsolekalendar --export-type csv 2>/dev/null
"mercredi 29 juillet 2020","","mercredi 29 juillet 2020","","test allday","Default location","","fea4c06b-f818-47b1-bbfe-0dc3eae9dbfb"
"mercredi 29 juillet 2020","08:00:00 EDT","mercredi 29 juillet 2020","09:00:00 EDT","test time","Default location","description","f605a223-b27f-4049-95bf-20b2deea67d4"

After:

$ LC_TIME=fr_FR.UTF-8 konsolekalendar --export-type csv 2>/dev/null
"2020-07-29","","2020-07-29","","test allday","Default location","","fea4c06b-f818-47b1-bbfe-0dc3eae9dbfb"
"2020-07-29","08:00:00","2020-07-29","09:00:00","test time","Default location","description","f605a223-b27f-4049-95bf-20b2deea67d4"