[Digital Clock] Replace 12/24hr tri-state checkbox in config UI with combobox

Summary:
The use of a tri-state checkbox for the three possible states (12hr, 24hr, use locale
default) is not ideal because it violates the convention regarding what tri-state
checkboxes are used for: nested lists where some sub-items can be unselected.

This patch replaces it with a combobox that clearly indicates all three states.

While we're at it, we add a button to open the Formats KCM in case people want to change their region.

BUG: 402487
FIXED-IN: 5.16.0

Test Plan:
Tested functionality with en_US locale. Check out the clock in the bottom-right corner of
the following screenshots:

Default state: "Use Region Defaults":

Force 12 hour time:

Force 24-hour time:

Reviewers: Plasma, Zren, VDG, davidedmundson, ndavis

Reviewed By: Plasma, VDG, davidedmundson, ndavis

Subscribers: cfeck, totte, ndavis, abetts, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D19230