libgimpwidgets: bring back GIMP_STOCK_EDIT as "gtk-edit".

Commit c6f1761 deletes a stock item id. But this is also part of our API
and cannot be deleted. Yet since the associated label ("_Edit") is
identical to the one for GTK_STOCK_EDIT, we can simply redefine it
to "gtk-edit".
Ideally we would also add a deprecation warning when this macro is used,
but I can't find a good way to add deprecation on macros.

libgimpwidgets/gimpicons.h

@@ -308,6 +308,9 @@ G_BEGIN_DECLS
 #define GIMP_STOCK_CONTROLLER_MOUSE         GIMP_STOCK_CURSOR
 #define GIMP_STOCK_CONVERT_PRECISION        GIMP_STOCK_CONVERT_RGB
 
+/* Deprecated icons: */
+
+#define GIMP_STOCK_EDIT                     "gtk-edit"
 
 GIMP_DEPRECATED_FOR(gimp_icons_init)
 void   gimp_stock_init           (void);