We can't add properties to the interface, since it breaks
3rd party implementations of the GtkFontChooser interface.
These exist, for example in gnumeric.
So, instead of a new property, add getter/setter vfuncs.
GtkFontButton already has a property named "font-name" which may
conflict / be an ABI break when moving GtkFontButton to implement
GtkFontChooser. Also, this is more in line with how other parts in
gtk (e.g. GtkCellRendererText) call a font string property.
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
