Also specify a default StyleName for fonts

Summary:
When saving, kcm_fonts adds the default selected StyleName,
so it needs to specified here as well to avoid mismatches.

BUG: 383191

Test Plan:
Ran pkcon install with no kdeglobals.
Without this change: Bold title in the auth dialog
With this change: Normal title in the auth dialog

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D7160

Cherry-picked from plasma-integration:1b21b5977c2068c5bd30c9f9f641f60bdba9ea8e
and slightly adapted for OS X

Committed from host : Portia.local