Prevent shadow post types being listed on Menu admin screen

Menu admin screen currently lists all post types, canonical and shadow.

So if we have 4 languages active in a basic WP site, there will be 4x lists of 'Posts', 4x lists of 'Tags', 4x lists of Categories.

Possibly related to #92 ?

Start at the WordPress Menu admin screen and trace how post types are collected for the menu UI. Reproduce the issue with four active languages, then verify that only canonical post types are listed and shadow post types no longer create duplicate Posts, Tags, or Categories lists.

Written by the indexing model from the issue text.