Allow for easier syncing of Plasma font

Summary:
Currently the user needs to have a fonts replacment file in `/.config/fontconfig/conf.d/` if they want to sync their Plasma font with SDDM.

That's not a good solution because users simply don't have this file or wouldn't know how to create it themselves.

In conjuction with https://github.com/sddm/sddm/pull/1191 this patch removes the need for additional user effort and allows for easy font syncing by writing the Plasma font value to the SDDM config file.

Test Plan: Dependency:  https://github.com/sddm/sddm/pull/1191

Reviewers: ngraham, #plasma, davidedmundson

Reviewed By: ngraham

Subscribers: davidre, plasma-devel

Tags: #plasma

Maniphest Tasks: T12710

Differential Revision: https://phabricator.kde.org/D23257

(cherry picked from commit 8171db27)

BUG: 432930
FIXED-IN: 5.21.1