Adapt to new Solr config :

- category -> genre
- cover in solr
- producer -> producerCode

lib/AudioBook.php

@@ -184,9 +184,9 @@ class AudioBook extends DbMapping
     }
 
     /**
-     * Retrieve the list of all categories
+     * Retrieve the list of all genres
      */
-    public static function listOfCategories()
+    public static function ListOfGenres()
     {
         $sql = "SELECT
                     LTRIM(RTRIM(Code)) as code,