RepositoryQuery: Ensure compatibility with the FilterEditor widget

refs #8826

library/Icinga/Repository/RepositoryQuery.php

@@ -65,6 +65,16 @@ class RepositoryQuery implements QueryInterface
         return $this;
     }
 
+    /**
+     * Return the columns to fetch
+     *
+     * @return  array
+     */
+    public function getColumns()
+    {
+        return $this->query->getColumns();
+    }
+
     /**
      * Set which columns to fetch
      *