Locked regions

Stop drops into part of the layout: a model.use middleware vetoes tab.move, tabset.move and tab.add into a region its tabs do not belong to (a drag asks it with model.can, so a refused target shows no outline), and droppable, draggable and splittable lock a tabset.

• model.use
• veto
• model.can
• typed data
• droppable
• draggable
• splittable
• Tooltip