KFontDialog lives in KDELibs4Support; it's used by:
Fonts KCM (from plasma-desktop)
kmplayer (but kmplayer itself is going to be deprecated)
- users can set double (11.3) font sizes (v.s. int only sizes in QFontDialog)
- font diff stuff, i.e. you can change more than one font settings in one go, e.g. "Adjust all fonts" in the fonts KCM; and being able to change just the family, style, or size, i.e. all three needn't be changed together
- setting a font to pre-select in the dialog, it'll select the correct "Regular"-like style even if the font doesn't have the styleName property set; and fonts with "Regular" styles shouldn't have that property set if we want setBold(true) to work correctly, see https://bugreports.qt.io/browse/QTBUG-63792 and https://bugs.kde.org/show_bug.cgi?id=378523
see https://phabricator.kde.org/D27808 for more details.