Add method for sorting by season/episode.

This makes season/episode sorting actually work correctly, while before,
clicking on the column to sort by it would instead fall through to a
title sort.

Fixes #10422

includes/sorting.php

@@ -145,6 +145,12 @@ function by_subtitle(&$a, &$b) {
         return by_no_articles($a->subtitle, $b->subtitle);
     }
 
+    function by_season(&$a, &$b) {
+        $asep = $a->season.'x'.str_pad($a->episode,2,"0",STR_PAD_LEFT);
+        $bsep = $b->season.'x'.str_pad($b->episode,2,"0",STR_PAD_LEFT);
+        return strcasecmp($asep, $bsep);
+    }
+
     function by_type(&$a, &$b) {
         return strcasecmp($a->texttype, $b->texttype);
     }