WinGui: Fix a bug in the GongSolutions.Wpf.DragDrop library that was causing queue items to be re-added after they were removed from the list simply by hovering the mouse over the control.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5203 b64f7644-9d1e-0410-96f1-a4d463321fa5