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/listener/ViewerHttpSessionListener.java
{
	/**
	 * After session created
	 * 
	 * @see javax.servlet.http.HttpSessionListener#sessionCreated(javax.servlet.http.HttpSessionEvent)
	 */
	public void sessionCreated( HttpSessionEvent event )
	{
		
		HttpSession session = event.getSession( );
		session.setAttribute( IBirtConstants.TASK_MAP, new HashMap( ) );
	}
	/**
	 * When session destroyed
	 * 
	 * @see javax.servlet.http.HttpSessionListener#sessionDestroyed(javax.servlet.http.HttpSessionEvent)
	 */
	public void sessionDestroyed( HttpSessionEvent event )
	{
		String sessionId = event.getSession( ).getId( );
		ParameterAccessor.clearSessionFiles( sessionId );
	}
}