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
{
		assert path != null;
		File file = new File( path );
		if ( !file.exists( ) )
			return file.mkdirs( );
		return true;
	}
	/**
	 * Returns the temp image folder with session id
	 * 
	 * @param request
	 * @return
	 */
	public static String getImageTempFolder( HttpServletRequest request )
	{
		String tempFolder = imageFolder;
		
		String sessionId = request.getSession( ).getId( );
		if ( sessionId != null )
			tempFolder = tempFolder + File.separator + sessionId;
		return tempFolder;
	}
	/**
	 * Clear the temp files when session is expired
	 * 
	 * @param sessionId
	 */
	public static void clearSessionFiles( String sessionId )
	{
		if ( sessionId == null )