Add a size argument to xfw_window_get_icon()

What do you think about it? It would allow to have better quality icons on Wayland, and on X11 it would at least offer a better compatibility with get_icon()/get_min_icon(). Maybe it's possible to do better via wnck_set_default_icon_size(), but that can always be done later (this can have side effects apparently: https://gitlab.gnome.org/GNOME/libwnck/-/merge_requests/10#note_583387).

The same would apply to XfwApplication that I am about to add (instead of XfwClassGroup mentioned by mail).