Add all of the registered font families per font when using fontconfig.

This allows users to be more specific about the fonts they specify with
a CSS 'font-family' attribute.  For example fc-list shows the following:

DejaVuSerifCondensed-Bold.ttf: DejaVu Serif,DejaVu Serif Condensed
BellCentennialStd-SubCapt.otf: Bell Centennial Std,Bell Centennial Std SubCaption

And, previously, only the first family listed would be entered into the Qt
font db.