Add an example for accelerators with manual image menuitems

Add an example for accelerators with manual image menuitems

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.