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
continue;
			
			if ( !parsedParameters.containsKey( paramName ) )
			{
				parsedParameters
						.put( paramName, DataUtil
								.getDisplayValue( this.defaultValues
										.get( paramName ) ) );
			}
		}
		return parsedParameters;
	}
	/**
	 * find the parameter handle by parameter name
	 * 
	 * @param paramName
	 * @return
	 * @throws ReportServiceException
	 */
	public ParameterHandle findParameter( String paramName )
			throws ReportServiceException
	{
		return BirtUtility.findParameter( this.reportDesignHandle, paramName );
	}
	/**
	 * find the parameter definition object by parameter name
	 * 
	 * @param paramName
	 * @return
	 */