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
/**
	 * URL parameter name that gives the reportlet id.
	 */
	public static final String PARAM_INSTANCEID = "__instanceid"; 
	/**
	 * URL parameter name to indicate whether need to run a reportlet.
	 */
	public static final String PARAM_ISREPORTLET = "__isreportlet";
	/**
	 * URL parameter name to indicate the export encoding.
	 */
	public static final String PARAM_EXPORT_ENCODING = "__exportEncoding";
	/**
	 * URL parameter name to indicate the CSV separator.
	 */
	public static final String PARAM_SEP = "__sep";
	/**
	 * URL parameter name to indicate if fit to page when render report as PDF.
	 */
	public static final String PARAM_FIT_TO_PAGE = "__fittopage";
	/**
	 * URL parameter name to indicate if pagebreak pagination only when render
	 * report as PDF.
	 */
	public static final String PARAM_PAGEBREAK_ONLY = "__pagebreakonly";
	/**
	 * Indentify the display text of select parameter
	 */
	public static final String PREFIX_DISPLAY_TEXT = "__isdisplay__"; 
	/**