Add 'app_create_post' and 'app_put_post' hooks in Atompub backend.

[znarfor]

wp-app.php is not extensible at the moment, by adding this hooks, a plugin may be able to do some extra processing once a post is updated or created.

Use case: an Atom client may use an extension not supported in Wordpress core. With this patch, we may be able to support it through a plugin. Sample plugin code following.

function app_handle_license($post_ID, $entry)
{
	foreach ($entry->links as $link) {
		if ($link['rel'] == 'license') {
			update_post_meta($post_ID, 'license', $link['href']);
		}
	}
}

add_action('app_create_post', 'bm_app_handle_post', 10, /* 2 arguments */ 2);

add_action('app_put_post', 'bm_app_handle_post', 10, /* 2 arguments */ 2);

Typo, it should be

add_action('app_create_post', 'app_handle_license', 10, /* 2 arguments */ 2);
add_action('app_put_post', 'app_handle_license', 10, /* 2 arguments */ 2);

New version of the patch:

• small WP coding style changes
• change names to prefixed with 'atompub_*' instead of 'app_*' (AtomPub has replaced APP as the standard name for AtomPub)
• moved the atompub_put_post action below the post update error check.

(In [10750]) Add hooks to allow plugins to extend AtomPub support. Fixes #8827 props znarfor and josephscott.