Posts, Post Types: Add default menu_icon values for built-in post types.

This allows the icons to be retrieved via `get_post_type_object()`.

Props m.usama.masood, michael.ecklund, SergeyBiryukov.
Fixes #38844.
Built from https://develop.svn.wordpress.org/trunk@47196


git-svn-id: http://core.svn.wordpress.org/trunk@46996 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/post.php

@@ -30,6 +30,7 @@ function create_initial_post_types() {
 			'capability_type'       => 'post',
 			'map_meta_cap'          => true,
 			'menu_position'         => 5,
+			'menu_icon'             => 'dashicons-admin-post',
 			'hierarchical'          => false,
 			'rewrite'               => false,
 			'query_var'             => false,
@@ -54,6 +55,7 @@ function create_initial_post_types() {
 			'capability_type'       => 'page',
 			'map_meta_cap'          => true,
 			'menu_position'         => 20,
+			'menu_icon'             => 'dashicons-admin-page',
 			'hierarchical'          => true,
 			'rewrite'               => false,
 			'query_var'             => false,
@@ -85,6 +87,7 @@ function create_initial_post_types() {
 				'create_posts' => 'upload_files',
 			),
 			'map_meta_cap'          => true,
+			'menu_icon'             => 'dashicons-admin-media',
 			'hierarchical'          => false,
 			'rewrite'               => false,
 			'query_var'             => false,