Added a shared GTK3 stock-to-icon-name helper in gtkutil and exposed it for reuse.

Updated gtkutil_button() to use icon-name images on GTK3 while preserving stock image usage on GTK2.
Switched menu icon creation to the shared GTK3 mapping helper.

src/fe-gtk/gtkutil.h

@@ -30,6 +30,9 @@ void gtkutil_destroy (GtkWidget * igad, GtkWidget * dgad);
 void gtkutil_destroy_on_esc (GtkWidget *win);
 GtkWidget *gtkutil_button (GtkWidget *box, char *stock, char *tip, void *callback,
 				 void *userdata, char *labeltext);
+#if HAVE_GTK3
+const char *gtkutil_icon_name_from_stock (const char *stock_name);
+#endif
 void gtkutil_label_new (char *text, GtkWidget * box);
 GtkWidget *gtkutil_entry_new (int max, GtkWidget * box, void *callback,
 										gpointer userdata);