support drag and drop sorting in the gallery preview

This implements jQuery sortables for the gallery post format tab, to set the `menu_order` for the images attached to the page. Drag and drop to order, an AJAX call is sent with the new order, then caught on the back-end and properties set accordingly.

Just like the last one - not doing Ridiculous Whitespace in order to keep the files here consistent. Sorry - maybe next time. :)