spinbutton: redesign to use an horizontal layout

Rewrite GtkSpinButton to use a touchscreen friendly horizontal layout.
Other features include:
- support for theming buttons with nth-child
- full support for RTL
- use + and - symbolic icons instead of arrows
- general streamlining and cleanup of a lot of related code

https://bugzilla.gnome.org/show_bug.cgi?id=663359

gtk/gtkspinbutton.h

@@ -207,7 +207,9 @@ gboolean        gtk_spin_button_get_snap_to_ticks  (GtkSpinButton  *spin_button)
 void            gtk_spin_button_update             (GtkSpinButton  *spin_button);
 
 /* private */
-GdkWindow*      _gtk_spin_button_get_panel         (GtkSpinButton  *spin_button);
+void            _gtk_spin_button_get_panels        (GtkSpinButton  *spin_button,
+                                                    GdkWindow     **down_panel,
+                                                    GdkWindow     **up_panel);
 
 G_END_DECLS