Allow setting of timezone when creating/changing local calendars

[guest4711]

When using calendar import/export or some local sync-methods (like myphone explorer) it would be useful to know which timezone the calendar belongs to.

When you create a local calendar the field "calendar_timezone" is left empty which means entries are floating / depend on local time. When switching from summer- to wintertime (e.g. CEST to CET) or moving from one timezone to another it's hard to decide wether an entry is correct or not (if not fixed by entering it in UTC or make it depending on the timezone).

So it would be nice to have the opportunity to enter/change the default timezone for local calendars.

[sefernet]

Meantime, a workaround is manually edit the ics file before the import...