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/MemberRef.java
Structure struct = getStructure( list, 0 );
				if ( struct == null )
					return null;
				
				assert member[0].isList( );
				list = (ArrayList) struct.getProperty( module, member[0] );
			}
			return list;
		}
		
		Structure struct = (Structure) element.getProperty( module, propDefn );
		if ( struct != null && member[0] != null && member[0].isList( ) )
			return (ArrayList) struct.getProperty( module, member[0] );
		return null;
	}
	/**
	 * Indicates whether this member reference points to a list.
	 * 
	 * @return true if points to a list.
	 */
	public boolean isListRef( )
	{
		switch ( refType )
		{
			case PROPERTY :
				return propDefn.isList( );
			case PROPERTY_MEMBER :
			case PROPERTY_LISTn_MEMBER :