Improvements for timezone list

Edited by @georgehrke:
tl;dr:

• offer up to 10 frequently used timezones on top of timezone list
• make timezone list searchable, because it's really long and hard to find specific timezones
• have a hidden list of cities / some other metric, to automatically map major cities to their timezone.
  (e.g. ignoring Europe/Busingen, there is only one timezone in germany Europe/Berlin. If you search for other major german cities like Hamburg, Munich, Cologne etc, search results should include Europe/Berlin)

also see https://github.com/nextcloud/calendar/issues/164 about introducing a world map for selecting a timezone.

Original issue:

Calendar 1.6.1
NextCloud 13.0.2

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Steps to reproduce

1. Create new event not in default time zone (Y) once, choose time zone (X) from long list of options
2. Create a new event again also in time zone X, search again.
3. Repeat

Expected behaviour

Tell us what should happen

I would find it more convenient if time zones I've selected previously (or frequently) were listed close to the default selection, this could be duplicated or out of order, but the list of all possible time zones would enumerate the list I've used previously first (or those that I've used frequently, or most recently, or 10 most recent, or 10 most frequent, all are acceptable and reasonable options and I'm sure there are more that would make sense).

This would save searching for frequently used time zones that are not local.

It is cool it defaults to local.

It is also cool that start and end are individually selectable.

Maybe i care too much about this: https://www.blackrosetech.com/gessel/2010/08/22/working-toward-workable-time-zones

Actual behaviour

The list is rather long and finding the intended time zone can be rather time consuming.

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Start by reviewing the timezone-list UI and the related world-map discussion in issue #164. The issue proposes three separate improvements—frequent timezones, search, and city-to-timezone mapping—so first establish which item is in scope and how completion will be verified.