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
* Indentify the parameter value is a locale string
	 */
	public static final String PREFIX_ISLOCALE = "__islocale__"; 
	/**
	 * URL parameter name to indicate the resource folder of all the report
	 * resources.
	 */
	public static final String PARAM_RESOURCE_FOLDER = "__resourceFolder"; 
	/**
	 * URL parameter name to indicate the target servlet pattern.
	 */
	public static final String PARAM_SERVLET_PATTERN = "__pattern"; 
	/**
	 * URL parameter name to indicate the target window.
	 */
	public static final String PARAM_TARGET = "__target"; 
	/**
	 * URL parameter name to indicate whether cache the parameter.
	 */
	public static final String PARAM_NOCACHE_PARAMETER = "__nocache"; 
	/**
	 * URL parameter name to indicate if open document as attachment(default is
	 * inline ).
	 */
	public static final String PARAM_AS_ATTACHMENT = "__asattachment"; 
	/**
	 * URL parameter name to indicate the execution name.
	 */
	public static final String PARAM_ACTION = "__action"; 
	/**