MDL-36874 mod_book: Add uarrow() output renderer.

In preparation for Book navigation improvements in MDL-36874, this commit
adds a uarrow() output renderer, to complement rarrow() and larrow().

lib/outputrenderers.php

@@ -3189,6 +3189,19 @@ EOD;
         return $this->page->theme->larrow;
     }
 
+    /**
+     * Accessibility: Up arrow-like character is used in
+     * the book heirarchical navigation.
+     * If the theme does not set characters, appropriate defaults
+     * are set automatically. Please DO NOT
+     * use ^ - this is confusing for blind users.
+     *
+     * @return string
+     */
+    public function uarrow() {
+        return $this->page->theme->uarrow;
+    }
+
     /**
      * Returns the custom menu if one has been set
      *