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
import java.util.PropertyResourceBundle;
import java.util.Set;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.http.HttpServletRequest;
import org.apache.axis.AxisFault;
import org.eclipse.birt.report.IBirtConstants;
import org.eclipse.birt.report.context.BaseAttributeBean;
import org.eclipse.birt.report.resource.BirtResources;
import org.eclipse.birt.report.resource.ResourceConstants;
/**
 * Utilites class for all types of URl related operatnios.
 * <p>
 */
public class ParameterAccessor
{
	
	/**
	 * URL parameter name that specifies the viewer id.
	 */
	public static final String PARAM_ID = "__id"; 
	/**
	 * URL parameter name that specifies the report title.
	 */
	public static final String PARAM_TITLE = "__title"; 
	/**
	 * URL parameter name that specifies whether show the report title.
	 */