Fix agenda/agendaupdate handling of event-specific time zones

[michaelmhoffman]

Currently, agendaupdate uses the calendar time zone when setting date and time for modified events. If the event has a different time zone, this will lead to date/time information being corrupted.

Probably the best solution is to include time zones in the agenda output for time details and parse it back in agendaupdate.

If time zone information is not included in the input, agendaupdate should read the event's current time zone before modifying it. This would require other API hits to fetch the current events. A naive approach would double the API hits. A better approach would do it once for all the input events but would require locally passing through it twice.