Locked regions

Stop drops into part of the layout: model.setOnAllowDrop refuses tabs that do not belong to a region, enableDrop, enableDrag and enableDivide lock a tabset, and onAction vetoes what gets through. Custom drop zones and feedback for refused drops are the Drop control Epic (#19).

• Model.setOnAllowDrop
• DropInfo
• enableDrop
• enableDrag
• enableDivide
• onAction
• Tooltip