Focus on specific post fields that have conflicts

[westonruter]

When there are post save conflicts due to concurrent editing of a post, the post fields get an error message indicating the their version of each conflicted field. In addition to showing the error message, we should focus on the first specific conflicted field (if the section is expanded), instead of just focusing on the first control regardless.

[westonruter]

The \WP_Customize_Post_Setting::is_post_data_conflicted() method should return an array of the fields that are conflicted, and then the sanitize method should return a WP_Error that has each setting_property identifying the property that is conflicted.