[applets/digital-clock] Switch timezones by clicking on them in the popup

Summary:
This patch allows you to switch the timezone by clicking on your desired one in the
applet's expanded representation. This is much simpler than the mouse-wheel-based UI and
does not need a config setting to turn it on or off.

Test Plan:

Reviewers: VDG, Plasma, jriddell, cblack

Reviewed By: VDG, Plasma, cblack

Subscribers: plasma-devel

Tags: Plasma

Maniphest Tasks: T9222

Differential Revision: https://phabricator.kde.org/D28133