* gtk/gtkseparator.[ch]: implement the GtkOrientable interface and
swallow all code from GtkHSeparator and GtkVSeparator. Add
gtk_separator_new() which takes a GtkOrientation argument.
* gtk/gtkhseparator.c
* gtk/gtkvseparator.c: remove all code except the constructor and
call gtk_orientable_set_orientation() in init().