'beforefeaturemodified' triggered when a feature is selected to be modified,
'featuremodified' triggered when a feature has been modified,
'afterfeaturemodified' triggered when a feature is finished being modified
r=tschaub (Closes#1358)
git-svn-id: http://svn.openlayers.org/trunk/openlayers@6413 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf