* gtk/gtkpaned.[ch]: implement the GtkOrientable interface
and swallow all code from GtkHPaned and GtkVPaned. Add
gtk_paned_new() which takes a GtkOrientation argument. Deprecate
gtk_paned_compute_position() for good (also for GTK_COMPILATION).
* gtk/gtkhpaned.[ch]
* gtk/gtkvpaned.[ch]: remove all code except the constructor and
call gtk_orientable_set_orientation() in init().