Missing documentation for set_always_show_image

[Mathieu Comandon]

A recent decision has been made from the Gnome Desktop to remove icons from the
menus.
I have nothing against this decision but apps writers like myself may want to
display an icon whatever the settings are.

I could not find any reference to gtk.ImageMenuItem.set_always_show_image(bool)
in the Reference Manual which is pretty severe given that the decision to
remove icons will affect a lot of users and application developers.

I've added the documentation to get and set always_show_image. I've also added
get and set use_stock which was missing.