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
*            http request
	 * @return report page range
	 */
	public static String getPageRange( HttpServletRequest request )
	{
		return getParameter( request, PARAM_PAGE_RANGE );
	}
	/**
	 * Get reportlet id from Http request.
	 * 
	 * @param request
	 *            http request
	 * @return reportlet id
	 */
	public static String getReportletId( HttpServletRequest request )
	{
		if ( isIidReportlet( request ) )
			return getParameter( request, PARAM_INSTANCEID );
		if ( isBookmarkReportlet( request ) )
			return getParameter( request, PARAM_BOOKMARK );
		return null;
	}
	/**
	 * Get named parameters from http request. parameter names and values are
	 * all in iso-8859-1 format in request.
	 * 
	 * @param request
	 *            incoming http request
	 * @param parameterName
	 *            parameter name in UTF-8 format