Make application_x11_get_icon() and window_x11_get_icon() consistent
Follow-up to discussion in https://gitlab.xfce.org/kelnos/libxfce4windowing/-/commit/8e53ee64b3f7935943ccd797ac5f0280c004ca23.
@kelnos I think this code should be shared, but I'm not sure where to put it or how to name it. Is it okay like this or do you prefer something else?