Consider workaround for Kalendar/libkcal undefined VTIMEZONE bug

[da4089]

See

• Cannot create new event from Kalendar  Kozea/Radicale#1236
• https://bugs.kde.org/show_bug.cgi?id=452656

KDE's Kalendar, and its underlying library libkcal, create iCalendar instances in which the VTIMEZONE component has a single TZID element (and nothing else).

While technically this is illegal according to IETF RFC-5545, we should consider a work-around in which vObject could look up the missing definition using the well-known Olsen database names (eg. US/Eastern).