pb browsing deeply embedded forums.

Most forums, like drupal.org's own forums, have one or several containers at the top level, and each containers contain several forums.

But if a site has very deeply embedded forums (forums within containers, within container, within containers, etc..) and at each level many containers and forums (more than your average web site), the page at ?q=forum quickly becomes unwieldy and hard to browse.

This patch displays forums up to a certain depth, and sub-forums are discovered when going into their parent containers/forums. The depth limit is set at 2 in this patch, but could be more, or could even be configurable.
This patch would not affect at all sites like drupal.org whose forums layout is simple (one level of containers, and one level of forums).

There is maybe a better, more complete approach (one that displays the mention: "there are %d sub-forums", like other popular forums do, e.g. phpBB3), but I didn't see any obvious, simple way to achieve this.