Added gtkutil helpers to create stock-based images and buttons while keeping GTK3 icon-name mapping centralized in gtkutil.c.

Exposed the new gtkutil stock helper APIs in the header for reuse.
Swapped setup dialog stock button creation to the new gtkutil_button_new_from_stock helper (including the sound play button fallback).

src/fe-gtk/gtkutil.h

@@ -31,6 +31,8 @@ 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);
+GtkWidget *gtkutil_image_new_from_stock (const char *stock, GtkIconSize size);
+GtkWidget *gtkutil_button_new_from_stock (const char *stock, const char *label);
 #if HAVE_GTK3
 const char *gtkutil_icon_name_from_stock (const char *stock_name);
 #endif