Support keyboard drag between vertical lists

[AbsoluteDestiny]

Description

Keyboard support for dragging items between lists requires lists to be horizontal as the next/prev list is determined by X position. Having other ways to determine the 'next' and 'previous' lists would improve keyboard accessibility. Some approaches could be:

• give lists an optional numeric index property that could be used to determine list order
• have a way to specify vertical list arrangement as on option
• fallback to y position when the lists have the same x position
• fallback to DOM order when lists have the same x position
• have a way to override a specific keyboard command to programmatically determine where the item should go next