DnD extension retains references to items

[neonstalwart]

in the DnD extension, insertRow adds items to the dndSource which are never cleared. dndSource retains a reference to these objects via it's map property and over time that will grow out of control. For stores where the size of the data represented by that store is large, the performance impact of these retained references can be considerable - even to the point of bringing the grid to a grinding halt.

some logic should be added to remove those items from dndSource at a suitable time.

this is possibly the cause of #318 although performance.html doesn't use DnD so perhaps not.