Bug 181121 Model should provide utility method for cube to get correspond dimension key.
In model cube handle, the relation of cube data set and dimension data sets are managed by a series of dimension condition handles.We need model to add utility method so that we can easily get corresponding dimension data set column names according to given cube dimension keys.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/core/Structure.java
*            the module
	 * @param element
	 *            the element contains this structure
	 * @return the semantic error list
	 */
	public List validate( Module module, DesignElement element )
	{
		return new ArrayList( );
	}
	/**
	 * Gets the specific handle of this structure. This structure must be in the
	 * element's structure list. The structure handle is transient because the
	 * position in the structure list is kept. The position changes if any
	 * structure is added, or dropped. So this handle should not be kept.
	 * 
	 * @param valueHandle
	 *            the value handle of this structure list property this
	 *            structure is in
	 * @param index
	 *            the position of this structure in structure list
	 * @return the handle of this structure. If this structure is not in the
	 *         <code>valueHandle</code>,<code>null</code> is returned.
	 */
	public StructureHandle getHandle( SimpleValueHandle valueHandle, int index )
	{
		if ( valueHandle == null || valueHandle.getListValue( ) == null )
			return null;
		if ( index < 0 || index >= valueHandle.getListValue( ).size( ) )
			return null;
		return handle( valueHandle, index );