Ensure that previewed changes apply on get_pages filter

[westonruter]

The call to get_pages() may be (rightly) cached because it is used to build an entire tree of all posts. However, because the value may be cached, it will not respected the current customized state, besides the fact that it doesn't use WP_Query to begin with.

• Call \WP_Customize_Posts_Preview::preview_setup_postdata() on each of the items in the get_pages filter.
• Ensure that newly-added pages are included and appear in the right sort order.
• Ensure newly added posts via nav menus are included (the auto-drafts among nav_menus_created_posts). See Ensure that post/page stubs created for nav menu items in WP4.7 can be edited #253.
• Beware of the $args for get_pages (other than post_type) and how get_pages would need to get re-applied. We have to initially support (per page_attributes_meta_box): post_type, exclude_tree, sort_column.

This is a dependency for #65