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
*            the element that has the property
	 * @param attr
	 *            the XML attribute name
	 * @param propName
	 *            the name of the property to write
	 */
	protected void attribute( DesignElement obj, String attr, String propName )
	{
		ElementPropertyDefn prop = obj.getPropertyDefn( propName );
		assert prop != null;
		Object value = obj.getLocalProperty( getModule( ), prop );
		if ( value == null )
			return;
		String xml = prop.getXmlValue( getModule( ), value );
		if ( xml == null )
			return;
		writer.attribute( attr, xml );
	}
	/**
	 * Writes one property entry of an element.
	 * 
	 * @param obj
	 *            the design element
	 * @param propName
	 *            the property name
	 */
	protected void property( DesignElement obj, String propName )
	{
		writeProperty( obj, null, propName, false );
	}