[applet/digital-clock] Show time zones in expanded representation too

Summary:
Right now time zones are only shown in the tooltip, which is annoying since the tooltip
times out aftew a few seconds, so it often closes before I can find the timezone I'm
looking for.

This patch adds a view for the current tomezones in the expanded representation, and
implements a mockup in https://bugs.kde.org/show_bug.cgi?id=395655 showing a proposal
for how to present this information.

The Expanded Representation is also ported to Layouts in order to make this task easier.

FEATURE: 395655
FIXED-IN: 5.19.0

Test Plan:
Just calendar:

Calendar + events:

Calendar + time zones:

Calendar + events + time zones:

Reviewers: VDG, Plasma, cblack

Reviewed By: VDG, cblack

Subscribers: cblack, apol, plasma-devel

Tags: Plasma

Maniphest Tasks: T9222

Differential Revision: https://phabricator.kde.org/D27903