Bug 213126 Add support for DataEngine-leve dataset cache
In many cases the DataEngine-leve dataset cache is useful which ensure that a dataset is executed one time during rendering one report. To support this feature a new dataset property should be added. The Birt customer can choose whether this dataset need to be cached or not by setting the property in deisgn time.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/writer/ModuleWriter.java
.getProperty( propName );
		PropertyDefn memberDefn = (PropertyDefn) propDef.getStructDefn( )
				.getMember( memberName );
		Iterator iter = list.iterator( );
		while ( iter.hasNext( ) )
		{
			IStructure struct = (IStructure) iter.next( );
			propertyWithoutName( struct, memberDefn.getName( ) );
		}
		writer.endElement( );
	}
	/**
	 * Writes the structure list, each of which has only one member.
	 * 
	 * @param obj
	 *            the design element
	 * @param propName
	 *            the name of the list property
	 */
	protected void writeStructure( DesignElement obj, String propName )
	{
		PropertyDefn prop = (ElementPropertyDefn) obj.getDefn( ).getProperty(
				propName );
		if ( prop == null )
			return;
		IStructure struct = (IStructure) obj.getLocalProperty( getModule( ),
				propName );
		if ( struct == null )
			return;
		writer.conditionalStartElement( DesignSchemaConstants.STRUCTURE_TAG );