libgimpwidgets: bring back GIMP_STOCK_EDIT as "gtk-edit".
Commit c6f1761a 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.