Gradebook headers have no tooltips, so long titles are hidden and unreadable (+ fix)

This has bugged me for a while.

Add this in place of line 844 of /grade/lib.php

$title = $element['object']->get_name();
$header .= $title;

And this in place of line 865:

$header = '<a href="'.$url.'" title="'.$title.'">'.$header.'</a>';