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
* report resources. True if only search the working folder, otherwise
	 * false.
	 */
	public static final String INIT_PARAM_WORKING_FOLDER_ACCESS_ONLY = "WORKING_FOLDER_ACCESS_ONLY"; 
	/**
	 * The parameter name that gives the repository lication to put the created
	 * documents and report design files.
	 */
	public static final String INIT_PARAM_DOCUMENT_FOLDER = "BIRT_VIEWER_DOCUMENT_FOLDER"; 
	/**
	 * Context parameter name that gives the absolute resource location
	 * directory.
	 */
	public static final String INIT_PARAM_BIRT_RESOURCE_PATH = "BIRT_RESOURCE_PATH"; 
	/**
	 * Context parameter name that gives preview report max rows limited.
	 */
	public static final String INIT_PARAM_VIEWER_MAXROWS = "BIRT_VIEWER_MAX_ROWS"; 
	/**
	 * Context parameter name that if always overwrite generated document file.
	 */
	public static final String INIT_PARAM_OVERWRITE_DOCUMENT = "BIRT_OVERWRITE_DOCUMENT"; 
	/**
	 * Context parameter name that defines BIRT viewer configuration file.
	 */
	public static final String INIT_PARAM_CONFIG_FILE = "BIRT_VIEWER_CONFIG_FILE"; 
	/**
	 * Context parameter name that if support print on the server
	 */
	public static final String INIT_PARAM_PRINT_SERVERSIDE = "BIRT_VIEWER_PRINT_SERVERSIDE";