add a method that does the equivalent of "refreshPositions : true"

right now if one has a dynamic setup where droppable and draggable are being used one has to use the refreshPositions option which is less performant since the possible drop targets are calculated on each drag-move.

In most cases that is not necessary since the programer can decide when the positions have to be refreshed. I assume that when the drag-process starts the possible drop target spaces are cached somewhere somehow. If one could have this cache rebuild by a method call this would be a very useful improvement.

example use case:

• a tree is displayed
• one drags a node in order to move it somewhere else
• while hovering the dragged node over another the other node opens revealing new potential drop targets
• now call "refreshPositions()" to make those newly revealed nodes potential drop targets