Bug 183919 [Enhancement] add a switch to determine whether to clean viewer image folder when session expires
in current viewer implementation, file in image folder is deleted when its corresponding session expires, could viewer add a switch to allow users to decide whether this deletion operation should be performed.
viewer/org.eclipse.birt.report.viewer/birt/WEB-INF/classes/org/eclipse/birt/report/context/ViewerAttributeBean.java
private Map defaultValues;
	/**
	 * Locale parameter list
	 */
	private List locParams;
	/**
	 * Constructor.
	 * 
	 * @param request
	 */
	public ViewerAttributeBean( HttpServletRequest request )
	{
		try
		{
			init( request );
		}
		catch ( Exception e )
		{
			this.exception = e;
		}
	}
	/**
	 * Init the bean.
	 * 
	 * @param request
	 * @throws Exception
	 */
	protected void __init( HttpServletRequest request ) throws Exception
	{