Stock icon stuff

[Alex Larsson <alexl redhat com>]

I just went through the stock icons, and there are a couple of problems
there.

stock_menu_sort_descending.png actually depics an ascending sort, and
there is no corresponding stock_menu_sort_ascending.png.

The definition:
#define GTK_STOCK_GO_UP            "gtk-up"
Is a bit odd, considering these definitions:
#define GTK_STOCK_GO_BACK          "gtk-go-back"
#define GTK_STOCK_GO_DOWN          "gtk-go-down"
#define GTK_STOCK_GO_FORWARD       "gtk-go-forward"

/ Alex