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
/**
	 * get parsed parameters as string.
	 * 
	 * @param parsedParameters
	 *            Map
	 * @param parameterList
	 *            Collection
	 * @param request
	 *            HttpServletRequest
	 * @param options
	 *            InputOptions
	 * 
	 * @return Map
	 */
	protected Map getParsedParametersAsStringWithDefaultValue(
			Map parsedParameters, Collection parameterList,
			HttpServletRequest request, InputOptions options )
			throws ReportServiceException
	{
		if ( parsedParameters == null )
			parsedParameters = new HashMap( );
		for ( Iterator iter = parameterList.iterator( ); iter.hasNext( ); )
		{
			
			ParameterDefinition parameter = (ParameterDefinition) iter.next( );
			if ( parameter == null )
				continue;
			
			String paramName = parameter.getName( );
			if ( paramName == null )