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
{
	/**
	 * Viewer preview max rows limited
	 */
	private int maxRows;
	/**
	 * Report parameters as string map
	 */
	private Map parametersAsString = null;
	/**
	 * Report parameter definitions List
	 */
	private Collection parameterDefList = null;
	/**
	 * Display Text of Select Parameters
	 */
	private Map displayTexts = null;
	/**
	 * Module Options
	 */
	private Map moduleOptions = null;
	/**
	 * Request Type
	 */
	private String requestType;
	/**
	 * Default parameter values map
	 */