Add default classes to quicktab links

Works fine by me, and really useful too. This functionality could be extended to add custom classes to links, and perhaps to the tabpage container.

While we wait for a maintainer to come through and review this and others, could I talk you into elaborating on this in the comments:
file: plugins/QuickQuicktabs.inc

    // Encode class suffixes longer than 50 as hash using md5.
    if (strlen($suffix) > 50) {
      $suffix = md5($suffix);
    }
 

It's a little unclear why 50 was decided on since the md5 space is technically smaller. Plus, referencing an md5 hash in css classes/id's will make the css unreadable. If this is to stay, maybe you could add and use a defined constant to indicate the max length, and optionally something that tests whether the developer wants to even convert to md5. Example:

define('QUICKTABS_MAX_SUFFIX', 50);

....
    // convert suffixes greater than QUICKTABS_MAX_SUFFIX to an MD5 hash.
    // if the QUICKTABS_MAX_SUFFIX constant is set to 0 in settings.php, this will be skipped entirely
    if (strlen($suffix) > QUICKTABS_MAX_SUFFIX && QUICKTABS_MAX_SUFFIX) {
      $suffix = md5($suffix);
    }

I'm going to put this in anyways. This is pretty valuable and needed. We can go through and clean this up later.

Thanks for the contribution!