AUI-1525 Make drag and drop keyboard accessible

Hi @brunobasto,

I've added keyboard support for the drag and drop. Below is an outline for the keyboard usage:
- Use `tab` key to navigate between draggable objects
- Press `ctrl` to trigger the drag operation
- While holding down `ctrl`, use left and right arrow keys to select a drop target
- Release `ctrl` to end dragging

Note: When `ctrl` key is first pressed, there's a slight delay before you can start using the arrow keys to choose a drop target.

I'm going to add implementation for the aui-accessible-dd-delegate. You can play around with the keyboard drag and drop with the aui-sortable-list module. Let me know what you think. Thanks!