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
* @return named parameters from http request
	 */
	public static Set getParameterValues( HttpServletRequest request,
			String parameterName )
	{
		return getParameterValues( request, parameterName, true );
	}
	/**
	 * Get report file name. If passed file path is null, get report file from
	 * request.
	 * 
	 * @param request
	 * @param filePath
	 * @return report file
	 */
	public static String getReport( HttpServletRequest request, String filePath )
	{
		if ( filePath == null )
			filePath = DataUtil
					.trimString( getParameter( request, PARAM_REPORT ) );
		
		if ( !isRelativePath( filePath ) )
			return filePath;
		
		if ( isRelativePath( workingFolder ) )
		{
			filePath = getRealPath( workingFolder + "/" + filePath, request 
					.getSession( ).getServletContext( ) );
		}
		else