Make look of Font Management KCM UI more consistent

Summary:
Apply the KDE HIG and make tooltips, buttons and others more consistent and better looking.
{F6618683}

Test Plan: Open the Font Management KCM.

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: ngraham, #vdg, plasma-devel, #plasma

Tags: #plasma

Maniphest Tasks: T10273

Differential Revision: https://phabricator.kde.org/D19079