Restore helper_alternate_class() function

This is a temporary measure to improve backwards compatibility of 1.3,
the function will be removed again in the next release.

Fixes #17522 (see also issue #17437)

core/helper_api.php

@@ -76,6 +76,28 @@ function helper_alternate_colors( $p_index, $p_odd_color, $p_even_color ) {
 	}
 }
 
+/**
+ * alternate classes for table rows
+ * If no index is given, continue alternating based on the last index given
+ * @param int $p_index
+ * @param string $p_odd_class default: row-1
+ * @param string $p_even_class default: row-2
+ * @return string
+ */
+function helper_alternate_class( $p_index = null, $p_odd_class = 'row-1', $p_even_class = 'row-2' ) {
+	static $t_index = 1;
+
+	if( null !== $p_index ) {
+		$t_index = $p_index;
+	}
+
+	if( 1 == $t_index++ % 2 ) {
+		return "class=\"$p_odd_class\"";
+	} else {
+		return "class=\"$p_even_class\"";
+	}
+}
+
 /**
  * Transpose a bidimensional array
  *