Add explicit ordering to the gallery shortcode

[koopersmith]

One of the media improvements we've discussed for 3.5 is using IDs in the gallery shortcode to specify the included attachments. We can do this currently with the include shortcode, but attachments are not sorted in the order specified, but by another metric (be it menu_order or otherwise). Since the gallery is specific to a given post, arranging attachments in the order the IDs were provided is a sensible improvement.

Maybe related: #13729?

Completely untested, and requires #13729 to be solved, which I'd really, really like.

21816.patch​ is an attempt at sorting in PHP (with a bit of cleanup).

posts_per_page wouldn't be an issue here, since get_posts() adjusts it accordingly:
http://core.trac.wordpress.org/browser/tags/3.4.1/wp-includes/post.php#L1450

In [21778]:

use 'ids' parameter for explicit attachment ordering in [gallery] shortcodes. fixes #21816

In 22891:

In the gallery shortcode, handle mapping of "ids" to include and orderby before the post_gallery filter. see #21816. fixes #22617.