Bug 137917 ImageHandle add setFile method.
According to new Resource Deployment Specification, image builder add a new image type 'File', so ImageHandle should add 'setFile' method as other type methods such as 'setURI'.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/ImageHandle.java
*            link, hyper-link, and drill through etc.
	 * @return a handle to the action property, return <code>null</code> if
	 *         the action has not been set on the image.
	 * 
	 * @throws SemanticException
	 *             if member of the action is not valid.
	 */
	public ActionHandle setAction( Action action ) throws SemanticException
	{
		setProperty( ImageItem.ACTION_PROP, action );
		if ( action == null )
			return null;
		return (ActionHandle) action
				.getHandle( getPropertyHandle( ImageItem.ACTION_PROP ) );
	}
	/**
	 * Returns the help text of this image item.
	 * 
	 * @return the help text
	 */
	public String getHelpText( )
	{
		return getStringProperty( ImageItem.HELP_TEXT_PROP );
	}
	/**
	 * Sets the help text of this image item.
	 * 
	 * @param helpText
	 *            the help text
	 *