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
* @see #getProperty
	 * @see #checkProperty
	 */
	public void setProperty( String propName, Object value );
	/**
	 * Performs a semantic check on the report item. The extended element can
	 * use this to validate the values of properties, to ensure that a set of
	 * values are consistent, etc.
	 * 
	 * @return List List contains ExtendedElementException.
	 */
	public List validate( );
	/**
	 * Creates deep copy of the IReportItem and return the new element.
	 * 
	 * @return the deep copied IReportItem
	 */
	public IReportItem copy( );
	/**
	 * Justifies whether the property list of the extended element is changed.
	 * 
	 * @return true if the property list of the extended element is changed,
	 *         otherwise false
	 */
	boolean refreshPropertyDefinition( );
	/**
	 * Returns the method definition list of extension elements. Each object in
	 * the list is instance of {@link IElementPropertyDefn}.
	 * 
	 * @return the method definition list.