The setter for the "font" property returned a boolean that indicated
whether the given font has been found. Instead, fall back to the default
font name when the given font doesn't exist.
Make the GtkFontChooser API similar to the Gtk{File,Recent,App}Chooser
APIs by introducing GtkFontChooser as an interface, that has a default
implementation in GtkFontChooserWidget.
https://bugzilla.gnome.org/show_bug.cgi?id=657627