ol-ext: Drag overlay

ol/interaction/Drag is an interaction to drag an Overlay over the map.

NB: the Overlay must be created with a stopEvent parameter set to false to propagate the events to the map.
NB2: ol/Overlay are not intended to be used on large data sets.

It can be applied to ol.Overlay, ol.Overlay.Popup or ol.Overlay.Placemark.

Move the overlays on the map...