Orphan preview holder when dragging is interrupted

After beginning a drag, if the mouse button is released outside of the screen, and if a subsequent drag operation is begun on another widget (while the previous widget still follows the mouse around), Draggable will trigger a new drag start event, causing Gridster to leave the current widget + its preview holder aside and start dragging the new widget. This will lead to an orphan preview holder to be left on the screen.

A quick solution is to ignore drag start without receiving a drag stop.