GtkButtonBox: remove spacing when buttons are linked

GtkButtonBox adds the "linked" class to its style context when its
layout is set to GTK_BUTTONBOX_EXPAND. It shouldn't ever make sense to
have spacing between buttons in that case, as themes generally draw
linked elements with a continuous border.

Thus, always set spacing to 0 and ignore GtkDialog's button-spacing
style property when the layout is set to EXPAND.

Also remove the now-redundant css rules which set button-spacing to 0
for message dialogs.

https://bugzilla.gnome.org/show_bug.cgi?id=752131

gtk/gtkdialog.c

@@ -696,8 +696,9 @@ update_spacings (GtkDialog *dialog)
       _gtk_box_set_spacing_set (GTK_BOX (priv->vbox), FALSE);
     }
 
-  gtk_box_set_spacing (GTK_BOX (priv->action_area),
-                       button_spacing);
+  /* don't set spacing when buttons are linked */
+  if (gtk_button_box_get_layout (GTK_BUTTON_BOX (priv->action_area)) != GTK_BUTTONBOX_EXPAND)
+    gtk_box_set_spacing (GTK_BOX (priv->action_area), button_spacing);
 
   if (!_gtk_container_get_border_width_set (GTK_CONTAINER (priv->action_area)))
     {