libgimp: gtk_image_new_from_stock() -> from_icon_name()
libgimpwidgets: add gimp_prop_icon_image_new()

and deprecate gimp_prop_stock_image_new().
libgimpwidgets: forgot a few image_new_from_stock() -> from_icon_name()
libgimpwidgets: use the renderer's icon-name in GimpIntComboBox