menu: Remove support for icon items from the window menu

We don't show these by default, and it uses deprecated API.
This also removes our only use of the stock icons, so remove
those as well.

https://bugzilla.gnome.org/show_bug.cgi?id=704437
Remove old, deprecated utilities that nobody has used in a million years

https://bugzilla.gnome.org/show_bug.cgi?id=704437