Add media workflow for choosing a custom background

[koopersmith]

This is a fairly simple workflow — select or upload an image and you're off to the races. We'll need to integrate this into both custom-background.php and the customizer. The code in custom-background.php may need some untangling.

See #21390 for the master media ticket.

[koopersmith]

Since the customizer doesn't already have access to the media modal and custom headers cannot use the modal without cropping, I don't think customizer integration with the media modal for custom backgrounds is a must have for 3.5.

[koopersmith]

In 22502:

Integrate media with the custom background page. fixes #21818, see #21390.

[koopersmith]

In 22503:

Custom Background: Remove dead code for thickbox. see #21818, #21390.