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
/**
	 * Context parameter name that gives the default locale of the BIRT viewer.
	 */
	public static final String INIT_PARAM_LOCALE = "BIRT_VIEWER_LOCALE"; 
	/**
	 * Context parameter name that gives the working folder of the local BIRT
	 * viewer user.
	 */
	public static final String INIT_PARAM_WORKING_DIR = "BIRT_VIEWER_WORKING_FOLDER"; 
	/**
	 * Context parameter name that gives the repository location of the image
	 * files.
	 */
	public static final String INIT_PARAM_IMAGE_DIR = "BIRT_VIEWER_IMAGE_DIR"; 
	/**
	 * Context parameter name that gives the repository location of the logging
	 * files.
	 */
	public static final String INIT_PARAM_LOG_DIR = "BIRT_VIEWER_LOG_DIR"; 
	/**
	 * Context parameter name that gives the logging level.
	 */
	public static final String INIT_PARAM_LOG_LEVEL = "BIRT_VIEWER_LOG_LEVEL"; 
	/**
	 * Context parameter name that gives the repository location of the script
	 * files to run reports.
	 */
	public static final String INIT_PARAM_SCRIPTLIB_DIR = "BIRT_VIEWER_SCRIPTLIB_DIR"; 
	/**
	 * Context parameter name that determines the search strategy of searching