These can't be returned as part of the font description,
so we need new api for them. For now, this is just readonly
properties. Maybe these should be writable too, eventually.
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.
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.
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
