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
*/
	private static String getRealPath( String path, ServletContext context )
	{
		assert path != null;
		String realPath = null;
		try
		{
			if ( !path.startsWith( "/" ) ) 
				path = "/" + path; 
			realPath = context.getRealPath( path );
			if ( realPath == null )
			{
				URL url = context.getResource( "/" ); 
				if ( url != null )
					realPath = DataUtil.trimString( url.getFile( ) ) + path;
			}
		}
		catch ( Exception e )
		{
			realPath = path;
		}
		return realPath;
	}
	/**
	 * Make directory
	 * 
	 * @param path
	 * @return
	 */
	private static boolean makeDir( String path )