Bug 179931 Provide new interface IStyle to let extension return the style properties values
Provide the IStyle interface to let extension user can return the style property value list for their extension elements.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/extension/IReportItem.java
* BIRT and its type is
	 * {@link org.eclipse.birt.report.model.metadata.XMLPropertyType}. The
	 * serialized data must be Unicode text. XML is preferred, but any text
	 * format is legal. If the model is binary, then one solution is to
	 * serialize the model as Base64 encoded data.
	 * 
	 * @param propName
	 *            the model property name to serialize
	 * @return a byte array that represents the serialized extended element
	 *         model.
	 */
	public ByteArrayOutputStream serialize( String propName );
	/**
	 * Deserializes the model. The property is visible to BIRT and its type is
	 * {@link org.eclipse.birt.report.model.metadata.XMLPropertyType}. The data
	 * provided will be that created by the <code>serialize</code> method. A
	 * well-written extension will handle two exceptional cases. First, it will
	 * handle all previous versions of this same extension. Second, it will
	 * handle erroneous input, perhaps created when a human editied the saved
	 * state by hand.
	 * 
	 * @param propName
	 *            the model property name to deserialize
	 * @param data
	 *            a byte array stream containing the serialized data
	 * @throws ExtendedElementException
	 *             if the serialized model is invalid
	 */
	public void deserialize( String propName, ByteArrayInputStream data )
			throws ExtendedElementException;