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 to indicate the client DPI setting.
	 */
	public static final String PARAM_DPI = "__dpi"; 
	/**
	 * Custom request headers to identify the request is a normal HTTP request
	 * or a soap request by AJAX.
	 */
	public static final String HEADER_REQUEST_TYPE = "request-type"; 
	/**
	 * The request type of "soap".
	 */
	public static final String HEADER_REQUEST_TYPE_SOAP = "soap"; 
	/**
	 * Parameter name that gives the IID of the export data form.
	 */
	public static final String PARAM_IID = "iid"; 
	/**
	 * Parameter name that gives the result set names of the export data form.
	 */
	public static final String PARAM_RESULTSETNAME = "ResultSetName"; 
	/**
	 * Parameter name that gives the selected column numbers of the export data
	 * form.
	 */
	public static final String PARAM_SELECTEDCOLUMNNUMBER = "SelectedColumnNumber"; 
	/**
	 * Parameter name that gives the selected column names of the export data
	 * form.
	 */
	public static final String PARAM_SELECTEDCOLUMN = "SelectedColumn";