Make tweaks to be able to implement sorting

* DragEvent now store mouseEvent with current position
* Add possibility to pass offset to DragImage.clone
* Add possibility to specify feedbackImage in DragSource constructor
Address @danschultz notes from #1

* Made _mouseEvent private, and add a getter to access it
* Not Dan's note, but: added _mouseMove, _mouseUp event checks for not
  null on global drag end, sometimes they may be null
Merge pull request #1 from astashov/tweaks-for-sorting

Make tweaks to be able to implement sorting