update Flex TreeGrid

Summary:
ÔÚDataGridÖÐÏÔʾ²ã¼¶½á¹¹£¬googleÁËһϣ¬ÕÒµ½flexlibÖеÄTreeGrid¡£°ÑÔ´Âë¼ÓÔص½ÏîÄ¿ÖУ¬°´ÕÕÎĵµÈ¥ÊµÏÖÆ书ÄÜ¡£×îºó·¢ÏÖÓкöàbug£¬±ÈÈ綨λ²»×¼£¬ÎÞ·¨Í϶¯µÈÎÊÌ⡣ͨ¹ýÔĶÁºÍÐÞ¸ÄTreeGridÀ࣬ÕâЩbug¶¼½â¾öÁË¡£½Ó×ÅÎÞÒâÖз¢ÏÖ£¬ÅÅÐòÓÐÎÊÌâ¡£µã»÷ÁÐÍ·ÅÅÐò£¬·¢Ïֲ㼶½á¹¹±ä»ìÂÒÁË£¬Õâ¸öÎÊÌâ±È½ÏÑÏÖØ¡£ÒòΪcreate dataProviderÊÇ×îºËÐĵÄÄÚÈÝÁË¡£ÔÚ¿ªÔ´ÉçÇø(Sapience360 )ÉÏÕÒµ½ÁËÁíÍâÒ»¸öTreeGrid,µ«°æ±¾ÓеãÀÏÁË£¬²»Ö§³Öflex2.0ÒÔÉÏ¡£
Requirement: ʹtreeGridÖ§³Öflex 2.0ÒÔÉÏ¡£
Solution:
¶Ô±ÈSDK 1.5 ºÍ SDK 3.2 DataGridÔ´Âë·¢ÏÖ£¬TreeGrid µÄplaceSortArrow() ·½·¨Ó¦¸ÃÐÞ¸Äһϡ£¿ÉÒÔÖ±½Óµ÷ÓÃDataGridHeaderÀàµÄ·½·¨¡£
 override protected function placeSortArrow():void
  {
DataGridHeader(header).mx_internal::_placeSortArrow();
  }
ɾ³ýÎÞÓõıäÁ¿ ÈçÏ£º
 /**
    *  @private
    *  The index of the column being sorted.
    */
   private var sortIndex:int = -1;

  
    *  @private
    *  The direction of the sort
    */
   private var sortDirection:String;

   /**
    *  @private
    *  The index of the last column being sorted on.
    */
   private var lastSortIndex:int = -1;
============================================== 
Ð޸ĺóµÄTreeGrid.as ÀàÈçÏ£º
package org.osflex {

import flash.display.DisplayObject;
import flash.display.Graphics;
import flash.display.Sprite;
import flash.events.Event;

import mx.collections.IViewCursor;
import mx.controls.DataGrid;
import mx.controls.dataGridClasses.DataGridColumn;
import mx.controls.dataGridClasses.DataGridHeader;
import mx.core.EventPriority;
import mx.core.FlexSprite;
import mx.core.IFlexDisplayObject;
import mx.core.mx_internal;
import mx.events.DataGridEvent;

use namespace mx_internal;

[IconFile("/assets/treegrid.png")]
/**
* A DataGrid that allow