This is somewhat tricky to work out, so put some example
code here for future reference. The tricky part is that
GtkAccelLabel tries to be smart about hiding the accel
if there's not enough space, so we have to make sure to
pack the label with expand=TRUE and set align=GTK_ALIGN_FILL,
or things won't work.
This test demonstrates that various deprecated ways
to construct buttons and menuitems still work as they
are supposed to, including always-show-image functionality.