AUI-1597 - aui-accessible-dd-delegate

Hi Drew,

I made some updates to the accessible-dd-delegate and accessible-dd-drag modules to get it to work with sortable-layout.  There are some things I'm not sure about, like the 'simulateMouseXY' logic, which we could possibly move into a 'sortable-layout-accessible' module.  I also removed some logic that I don't think was needed, as almost the same code exists in accessible-dd-drag.  

I also cherry-picked a commit from @marclundgren for the accessible-dd demo.  That way it's easy to test changes across modules.

Please let me know your thoughts.

Thanks!