refactor: replace LSL_Api.InventoryKey(string) largely with SceneObjectPartInventory.GetInventoryItem(string)

Also gets llStopAnimation() to call KeyOrName rather than duplicating logic.

OpenSim/Region/Framework/Interfaces/IEntityInventory.cs

@@ -163,6 +163,19 @@ namespace OpenSim.Region.Framework.Interfaces
         /// </returns>
         List<TaskInventoryItem> GetInventoryItems();
 
+        /// <summary>
+        /// Gets an inventory item by name
+        /// </summary>
+        /// <remarks>
+        /// This method returns the first inventory item that matches the given name.  In SL this is all you need
+        /// since each item in a prim inventory must have a unique name.
+        /// </remarks>
+        /// <param name='name'></param>
+        /// <returns>
+        /// The inventory item.  Null if no such item was found.
+        /// </returns>
+        TaskInventoryItem GetInventoryItem(string name);
+
         /// <summary>
         /// Get inventory items by name.
         /// </summary>