Break up post settings into post_field settings

[westonruter]

Currently posts are modeled by WP_Customize_Post_Setting in a similar way to how nav menu items are modeled: all of the fields are stored in an object value. This has some advantages, especially in regard to handling saves, but it makes it a lot harder to work with in terms of re-using existing controls which expect settings with scalar values and routing notifications to specific controls, including for conflict resolution.

Splitting up the post into multiple settings has been implemented in the Customize Featured Content demo plugin, and overall it felt much less hacky. I think we should consider refactoring to do the same in Customize Posts, refactoring WP_Customize_Post into a WP_Customize_Post_Field_Setting class.