[Fonts KCM] disable "to" label when its companion controls are also disabled

Summary: Title and screenshots say it all.

Test Plan:
Before:
{F5918418}

After:
{F5919513}

Reviewers: #vdg, davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D13611