prototype.js, this effectively replaces most of prototype's Event object with
the OpenLayers.Event object, which has undesirable results (i.e. calling
Event.observe is actually calling OpenLayers.Event.observe)." Fixed by
only creating Event is Event does not already exist. (Closes#880)
git-svn-id: http://svn.openlayers.org/trunk/openlayers@4040 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf