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. The K4TimeZoneWidget from KDELibs4Support is integrated and QTimeZone can be used for most of it, the only thing missing are the translations for continents and cities (Europe/Warsaw -> Europa/Warschau). The digitalclock applet which also deals in timezones has the same problem and there we maintain a list of translated names. Its translation catalog is used to obtain these here until a better solution for this comes to light. BUG:470611