Improve time zone selection UX

UI to pick a time zone in Tor Connection does not feel great to me: there's no way to search and the list is huge.

Could we instead use a popover with a search entry, like e.g. we have for the language in the Welcome Screen?

Implementation notes wrt. switching to a popover with search:

• The Welcome Screen uses the tailsgreeter.ui.popover helper library, which looks like it could be resulted as-is (and possibly moved to tailslib).

• Regarding usage, I admit it needs a fair bit of code but it does not look like rocket science and we have good examples, such as config/chroot_local-includes/usr/lib/python3/dist-packages/tailsgreeter/ui/region_settings.py.

Implementation ideas by increasing cost and UX benefit:

1. Split the list into 2: 1 combo for the continent, 1 combo for the city.
2. Allow jumping ahead in each of the lists by pressing characters on the keyboard.
3. Popover with a search entry like in the Welcome Screen.
4. World map, like in GNOME Date & Time but without the list of cities as it creeps out people.

When one of these options is implemented, please create GitLab issues for each of the remaining options with more cost and more UX benefit. But I don't want to wait years until we have the nicest one, if we can implement the first one earlier.