[Digital Clock] Use a smaller font size in horizontal panel mode

This patch makes the digital clock use a smaller font when used in a
horizontal form factor.

BUG:375969

Test Plan: Play with the widget settings in different panels/desktop

Reviewers: #plasma, #vdg, romangg

Reviewed By: #plasma, romangg

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6764