kcms/dateandtime: Port the date and time kcm to KF6

Aside from the usual kf6/kcm porting and using KSvg instead of
plasma-framework the meat is in migrating away from the KDE4-era
KTimeZone classes QTimeZone can be used for most of it, the only
thing missing are the translations for continents and cities
(Europe/Warsaw -> Europa/Warschau).
We can use ICU to retrieve a city name. The digitalclock applet which
also deals in timezones  there we maintain a list of translated contintent
names. Its translation catalog is used to obtain these here until a better
solution for this comes to light.
BUG:470611