add a new 'playing' property to the Tween class so that a call to stop doesn't do anything if animation is already finished, r=ahocevar,crschmidt (Closes #1392)

git-svn-id: http://svn.openlayers.org/trunk/openlayers@6387 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Map.js

@@ -318,6 +318,12 @@ OpenLayers.Map = OpenLayers.Class({
      *           Default is to fall through.
      */
     fallThrough: true,
+    
+    /**
+     * Property: panTween
+     * {OpenLayers.Tween} Animated panning tween object, see panTo()
+     */
+    panTween: null,
 
     /**
      * Constructor: OpenLayers.Map