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/utility/ParameterAccessor.java
*/
	public static final String PROP_BASE_URL = "base_url"; 
	/**
	 * Engine supported output formats
	 */
	public static String[] supportedFormats = {PARAM_FORMAT_HTML,
			PARAM_FORMAT_PDF};
	/**
	 * Flag that indicated if support print on the server side.
	 */
	public static boolean isSupportedPrintOnServer = true;
	/**
	 * Get bookmark. If page exists, ignore bookmark.
	 * 
	 * @param request
	 * @return the bookemark
	 */
	public static String getBookmark( HttpServletRequest request )
	{
		int page = getParameterAsInt( request, PARAM_PAGE );
		return page < 1
				? getReportParameter( request, PARAM_BOOKMARK, null )
				: null;
	}
	/**
	 * Get query string with new parameter value.
	 * 
	 * @param request
	 *            http request
	 * @param name