Sortable - Dragging several items

A drag carries one item, and that item can be a list. Tick some cards and drag one of them: its ItemFactory hands over the whole selection when the drag starts, IsDraggingOverride marks every selected card as dragged, and the archive receives them all. A card that is not ticked drags alone. The cards set UseCustomDragLayer, and a BdndDragLayer draws the badge that follows the pointer - "3 cards" - in place of the browser's picture of the one card that was grabbed. The keyboard does the same: Space on a ticked card picks the selection up (the layer asks Monitor.GetDragInput() and draws nothing for a keyboard drag, which has no pointer to follow).