Fix #14

Fix #14 
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Under certain conditions the 'dragend' event is triggered without having fired the 'drop' event
and 'event.dataTransfer.dropEffect' is set to a value other than 'none',
which causes the dndMove run without having previously copied element.
This causes the elements of the list are missing.
To fix dndDropEffectWorkaround.dropEffect is always used to store the dropEffect.