[simple-scan] Stop using deprecated Gtk.Arrow

[Robert Ancell <rancell src gnome org>]

commit db37c1c94b498bb2f96f195d8b81d651103f1619
Author: Robert Ancell <robert ancell canonical com>
Date:   Mon May 21 09:31:07 2018 +1200

    Stop using deprecated Gtk.Arrow

 src/app-window.vala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/app-window.vala b/src/app-window.vala
index 1475e0a..a62ca61 100644
--- a/src/app-window.vala
+++ b/src/app-window.vala
@@ -1068,7 +1068,7 @@ public class AppWindow : Gtk.ApplicationWindow
         {
             if (items[i] == '-')
             {
-                var a = new Gtk.Arrow (Gtk.ArrowType.RIGHT, Gtk.ShadowType.NONE);
+                var a = new Gtk.Label ("➤");
                 a.visible = true;
                 box.pack_start (a, false, false, 0);
                 page_box = null;