Hierarchical Select implementation for CCK

A couple of remarks:
- Since CCK itself does not have a strict hierarchical format to represent an item *uniquely* in such a hierarchy, we generate one on-the-fly.
- I've put the implementation in a separate file, content_hierarchical_select.inc, because it doesn't really belong anywhere else
- This file is included in hook_menu()'s uncached path. You can't just include it when you need it, because I also need to include it on JSON callbacks. If you don't like this, I could make HS automatically look for a "<modulename>/<modulename>_hierarchical_select.inc" file and include it on
the fly.
- I'm still working with merlin to get the CSS/JS loaded up front (through a new, yet to be written hook in Panels 2), Drupal.settings will also be handled through Panels, in a similar manner as in Views 2.
- Because of the previous remark, not everything that's necessary for HS, is loaded. HS degrades gracefully (albeit less gracefully without the CSS), so it stays functional. However, the "Add pane" button stops working, for an unknown reason (haven't had the time to track down the cause).
- You *can* use it properly in the demo module I attached.

So, summarized:
- the implementation of the HS API works fine.
- for actually using this with Panels, you may want to wait (due to the modal).
- you *can* use HS as a "progressive enhancement" elsewhere in the CCK UI, as long as you're not doing it in a pop-up. (I haven't looked where this could be useful, just saying that you can.)