Bug 160429 Drag the datasource and dataset and produce an error gradation in data explorer
Details: Drag the datasource and dataset and produce an error gradation in data explorer Step: 1.New 4 datasource named d1, d2, d3, d4. 2.Drag the datasource in data explorer to change the gradation. (1) Drag d1 between d2 and d3. (2) Drag d2 between d3 and d4. (3) Drag d4 between d2 and d3. (4) Drag d4 to the top. (5) Drag d3 between d1 and d2. (6) Drag d3 to the top. Actual result: (1) d1 will be move to bottom. (2) d2 will be move to bottom. (3) d4 will not be move to anywhere. (4) d4 will not be move to anywhere. (5) d3 will be move to bottom. (6) d3 will be move to bottom. Excepted result: The datasource will be move to the correct station.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/ModuleHandle.java
* @throws StyleSheetException
	 *             thrown if the resource is not found, or there are syntax
	 *             errors in the resource
	 */
	public CssStyleSheetHandle openCssStyleSheet( String fileName )
			throws StyleSheetException
	{
		StyleSheetLoader loader = new StyleSheetLoader( );
		return loader.load( module, fileName ).handle( module );
	}
	/**
	 * Gets the result style sheet with given file name of an external CSS2
	 * resource.
	 * 
	 * @param is
	 *            the input stream of the resource
	 * @return the <code>CssStyleSheetHandle</code> if the external resource
	 *         is successfully loaded
	 * @throws StyleSheetException
	 *             thrown if the resource is not found, or there are syntax
	 *             errors in the resource
	 */
	public CssStyleSheetHandle openCssStyleSheet( InputStream is )
			throws StyleSheetException
	{
		StyleSheetLoader loader = new StyleSheetLoader( );
		return loader.load( module, is ).handle( module );
	}
	/**
	 * Imports the selected styles in a <code>CssStyleSheetHandle</code> to