gtk.ImageMenuItem added.

Hi,

I added support for [GtkImageMenuItem](http://www.gtk.org/api/2.6/gtk/GtkImageMenuItem.html).

I couldn't find documentation for these functions, so I didn't add them:

* `gtk_image_menu_item_get_use_stock`
* `gtk_image_menu_item_set_use_stock`
* `gtk_image_menu_item_get_always_show_image`
* `gtk_image_menu_item_set_always_show_image`
* `gtk_image_menu_item_set_accel_group`