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
* @return
	 */
	public static String getExtensionName( String format )
	{
		if ( format == null )
			return null;
		String key = "viewer.extension." + format; 
		return DataUtil.trimString( getInitProp( key ) );
	}
	/**
	 * Returns the base url defined in config file
	 * 
	 * @return
	 */
	public static String getBaseURL( )
	{
		String baseURL = getInitProp( PROP_BASE_URL );
		if ( baseURL != null && baseURL.length( ) > 0 )
		{
			if ( baseURL.endsWith( "/" ) ) 
				baseURL = baseURL.substring( 0, baseURL.length( ) - 1 );
		}
		return baseURL;
	}
	/**
	 * convert path from System Properties Definition. For example:
	 * ${java.io.tmpdir}
	 * 
	 * @param path
	 * @return