styleschemechooserwidget: use block font
styleschemechooserwidget: use block font

This removes the C-isms in this widget which could be offputting and
confusing to users. Instead we use our BuilderBlocks font inherited from
Builder and use the label to describe the scheme name.

GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview