Media: API to add back the description field in the new media popup

[batmoo]

Description was dropped as a field in the media popup. Would be nice to have a allow plugins to re-add the field.

[nacin]

With the patch:

add_filter( 'attachment_fields_to_edit', function( $fields, $post ) {
	$fields['post_content'] = array(
			'label'      => __('Description'),
			'value'      => $post->post_content,
			'input'      => 'textarea'
	);
	return $fields;
}, 10, 2 );

[nacin]

On why the unset() calls are there: get_attachment_fields_to_edit() returns all of those fields: align, caption, size choices, etc. But because it returns all of those fields, it is very slow. (In particular, image_downsize() is slow. See #22598 for how we work around that.) So instead, we just apply the raw filter.

I kept the unset() calls because it is possible that a plugin could be re-defining some of those fields in a bad way (rather than just appending to the array), and we wouldn't want that, as we already handle those fields in newer, better ways.

The exception is Description (post_content). We don't handle it, but it was something get_attachment_fields_to_edit() previously returned. Stop unsetting it, and now a plugin can add it back with just a few lines of code.

[nacin]

Second patch tosses in a save handler for free. Since it used to be a default field, I figure we can just continue to account for it in core.

[batmoo]

Patch and code snippet work nicely for the media popup but duplicates the description field on the "Edit Media" screen.

[batmoo]

Working beautifully.

[koopersmith]

Looks good.

[ryan]

In 22929:

Introduce API to add back the description field in the new media popup.

Props nacin
fixes #22642