Add the Actionable interface.

	* tools/extra_defs_gen/generate_defs_gtk.cc: Add the GtkActionable
	GType to the extra defs generation utility so that its properties are
	generated.
	* gtk/src/gtk_signals.defs: Regenerate to get the properties of the
	new interface.
	* gtk/src/gtk_vfuncs.defs: Add the virtual function definitions.
	* gtk/src/gtk_methods.defs: Regenerate to get the methods for the new
	interface.

	* gtk/src/actionable.{ccg,hg}: Add the sources wrapping the methods,
	properties and virtual functions.
	* gtk/src/filelist.am: Include the sources in the list of sources to
	be built.
	* gtk/src/gtk_extra_objects.defs: Add the new interface to the list
	extra objects so that there no missing object definition warnings from
	gmmproc.

	Bug #637445.