Add live JS updates to post titles while waiting for selective refresh

[westonruter]

We can do instant previews of post titles by injecting the post title into the .entry-title via JS as a low-fidelity preview until we wait for the selective refresh response to return. This is similar to what has been implemented for blogname and blogdescription in 4.5.

[westonruter]

This almost does it:

--- a/js/customize-post-field-partial.js
+++ b/js/customize-post-field-partial.js
@@ -41,6 +41,22 @@
        /**
         * @inheritdoc
         */
+       preparePlacement: function( placement ) {
+           var partial = this, setting = api( partial.settings()[0] ), a;
+           if ( 'post_title' === partial.params.field_id && setting ) {
+               a = placement.container.find( 'a' );
+               if ( a.length ) {
+                   a.text( setting().post_title );
+               } else {
+                   placement.container.text( setting().post_title );
+               }
+           }
+           api.previewPosts.Partial.prototype.preparePlacement.call( partial, placement );
+       },
+
+       /**
+        * @inheritdoc
+        */
        showControl: function() {
            var partial = this, settingId = partial.params.primarySetting;
            if ( ! settingId ) {

The problem is that preparePlacement isn't called when a selective refresh request is currently open, so it only does a live JS update for the first change.