Improve the look of the Fonts KCM UI

Summary:
Makes some improvements to the Fonts KCM UI, like applying the KDE HIG, adding icons and tooltips to buttons and improving the buttons, updating strings and more.

Test Plan: Open the Fonts KCM.

Reviewers: Plasma, VDG, ngraham, rooty, davidedmundson

Reviewed By: Plasma, VDG, ngraham, rooty, davidedmundson

Subscribers: davidedmundson, ndavis, ngraham, rooty, plasma-devel, VDG, Plasma

Tags: Plasma

Maniphest Tasks: T10273

Differential Revision: https://phabricator.kde.org/D18861