Returns font files grouped in dict by font family.

def _groupFontByFamily(self, font_files):
        """
        Returns font files grouped in dict by font family.
        """
        font_families = defaultdict(list)
        for font_file in font_files:
            font = fc.FcFont(font_file)
            font_families[font.family[0][1]].append(font_file)
        return font_families