Add control to manipulate the post parent

[westonruter]

Fixes #65.

Continuation of #189.

• Address PR feedback from [WIP] Add control to manipulate the post parent #189.
• Implement get_pages() support in Ensure that previewed changes apply on get_pages filter #241 via Add support for including previewed changes in calls to get_pages() #250.
• Critically \WP_Customize_Posts::get_post_parent_choices() is only returning page posts. We would need to get all posts of all post types for this to work. At the very least this data needs to be loaded via Ajax, but likely what should be done is:
• Make the Customize Object Selector plugin a hard dependency, and introduce a “hierarchical post dropdown” control either in that plugin or in Customize Posts.
• Show indentation for pages which wp_dropdown_pages() provides, a deficiency with \WP_Customize_Posts::get_post_parent_choices().
• Merge Add support for emulating wp_dropdown_pages() when the post type is hierarchichal wp-customize-object-selector#16 to add wp_dropdown_pages support to the selector control.
• Add support for page_attributes_dropdown_pages_args filter (this really only makes sense if using object_selector). This could also be implemented later, just as implementing the page-specific templates (theme_page_templates filter) is not yet done.
• Add support for syncing the customizer value with the parent frame if opened from edit post screen. This can also be deferred for later, since there are other settings which aren't yet implemented.

Other items postponed for another time:

• Use object selector component with dropdown pages for Jump to Post? Or load stubs for all pages in WP? (There should also be the first page_for_posts auto-loaded for non-hierarchical posts.) See List all posts of a given post type in the section #146.
• Implement selective refresh for updating a nav menu that contains a post inside of it? See ea425d4