Prevent shadow post types being listed on Menu admin screen

[imsimond]

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 ?

[markoheijnen]

I guess we also should set our own property on those. So you can use that to filter it out when using get_post_types() for example