[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