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
* 
	 * @param s
	 *            string to be encoded.
	 * @param format
	 *            encoding format.
	 * @return
	 */
	public static String urlEncode( String s, String format )
	{
		String encodedString = s;
		if ( s != null )
		{
			try
			{
				encodedString = URLEncoder.encode( s, format );
			}
			catch ( UnsupportedEncodingException e )
			{
				encodedString = s;
			}
		}
		return encodedString;
	}
	/**
	 * Parse config file name from report design filename.
	 * 
	 * @param reportDesignName
	 *            String
	 * @return String
	 */