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
* groups within the report. The order that the items appear within the slot
	 * determines the order in which they appear in the "requester" UI.
	 * 
	 * @return A handle for working with the parameters and parameter groups.
	 */
	public SlotHandle getParameters( )
	{
		return getSlot( PARAMETER_SLOT );
	}
	/**
	 * Returns a cascading parameter group handle with the given group name
	 * 
	 * @param groupName
	 *            name of the cascading parameter group.
	 * @return a handle to the cascading parameter group. Returns
	 *         <code>null</code> if the cascading group with the given name is
	 *         not found.
	 */
	public CascadingParameterGroupHandle findCascadingParameterGroup(
			String groupName )
	{
		DesignElement element = module.findParameter( groupName );
		if ( element == null || !( element instanceof CascadingParameterGroup ) )
			return null;
		return (CascadingParameterGroupHandle) element.getHandle( element
				.getRoot( ) );
	}
	/**
	 * Returns a slot handle to work with the styles within the report. Note
	 * that the order of the styles within the slot is unimportant.