This commit adds constructors for GtkPaned, GtkBox, GtkButtonBox,
GtkRuler, GtkScale, GtkScrollbar and GtkSeparator and makes these
types instantiable.
* Due to object ownership issues it is impossible to correctly use
get_group/set_group from a GI binding
* join_group is safer because at the binding level it works with individual
GtkRadioAction objects and not with the list of objects that gets
modified in the library
