Added a new interface to extend layout information of GtkCellRenderers,
GtkCellRenderer is a prerequisite of GtkExtendedCellIface and provides
a base implementation of ->get_desired_size() with ->get_size().
Note that the api for GtkExtendedCell differs from GtkExtendedLayout
since the cells need to be passed an additional widget to calculate
sizes contextual to the widget/screen (fonts etc).
