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
*/
	protected static String convertSystemPath( String path )
	{
		if ( path == null )
			return path;
		
		Pattern p = Pattern.compile( "\\$\\s*\\{([^\\}]*)\\}\\s*(.*)", 
				Pattern.CASE_INSENSITIVE );
		Matcher m = p.matcher( path );
		if ( m.find( ) )
		{
			String sysPath = DataUtil.trimSepEnd( System.getProperty( m.group(
					1 ).trim( ) ) );
			return DataUtil.trimString( sysPath ) + m.group( 2 ).trim( );
		}
		return path;
	}
	/**
	 * Process working folder setting. If path is a relative path, first
	 * relative to context.
	 * 
	 * @param context
	 * @param path
	 * @return
	 */
	public static String processWorkingFolder( ServletContext context,
			String path )
	{
		path = convertSystemPath( DataUtil.trimString( path ) );
		String realPath = null;