[applets/digital-clock] Port timezone config UI to Kirigami.BasicListItem

At the time this custom delegate was implemented, Kirigami.BasicListitem didn't have the ability to easily display custom items inside itself. Now it does, so we can port to that to save some code and increase consistency.

The appearance is virtually identical and the UX remains the same with the exception that you can now select an item by clicking anywhere on the background, not just on one of the radio buttons.