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
* Checks the value of a property prior to setting it. BIRT calls this
	 * operation before setting the value. See the <code>setProperty</code>
	 * method for details on the process for setting a property value.
	 * 
	 * @param propName
	 *            the name of the property to check
	 * @param value
	 *            the new property value as one of the supported BIRT types
	 * @see #getProperty
	 * @throws ExtendedElementException
	 *             if the property value is invalid or the property name is
	 *             invalid
	 */
	public void checkProperty( String propName, Object value )
			throws ExtendedElementException;
	/**
	 * Sets the value of a property. This operation is done in response to a
	 * BIRT-defined command. That is, this method call is the result of an
	 * application using the BIRT Model API to set a property. BIRT will have
	 * validated the property using the <code>checkProperty</code> method, and
	 * will have created a command to set the property.
	 * <p>
	 * If this element supports styles or inheritance, then the value can be
	 * <code>null</code>, which means to clear the property value so that it
	 * will inherit from the base element or the style.
	 * 
	 * @param propName
	 *            the name of the property to set
	 * @param value
	 *            the new property value as one of the supported BIRT types