* Currently nothing happens in this case, because we insert the dragged item
after the target.
* On balance it seems more intuitive to swap the items in this particular case
even if it is slightly against the regular drag operations.
* Initial WIP
* Identical results with spreadsheet
* Improved and more robust smoothing by filtering points first
* Improved smoothing and more GUI
* Include mixed-label for smoothing threshold
* Implement WBS parameters track
* Much better alignment of tracks without flickering
* Don't replace user description when dragging and dropping tracks
* Fix missing OBG and SH curves
* Fix curve and track D&D
* Make sure parameter curves aren't WBS curves
Getting drag items from selection when the drag is started (when the
interface asks for supported drop actions). Using proposed the drop
action along with the drag items to determine if the drag item can be
dropped onto a given target in the flags() method.
