Integrate with Object Selector Control to include an Add New button

[westonruter]

The Customize Object Selector Control provides a UI for selecting one or more posts via Select2.

If the user wants to add a post that doesn't exist yet, there should be a button to be able to add a new post instead of the user being blocked. This is very similar to what is described in being able to add new posts when selecting a nav menu item to add: https://core.trac.wordpress.org/ticket/34923

Clicking an “Add New” button should create the post of that type and then focus on the new section for that post type. When clicking collapse on the new post Customizer section, the user should be returned to previous Customizer section with the selection control focused and the newly-added item appearing in the list. See Customizer UI breadcrumbs as implemented for https://core.trac.wordpress.org/ticket/32683

Depends on #48.
Depends on #50.
Depends on #134.

Originally at danielbachhuber/customizer-ajax-select#5

[westonruter]

Done over in xwp/wp-customize-object-selector#2