Add forum setting to limit subforums legend to direct children only

Adding an option for each forum that will limit the number of subforums that are displayed in the forum's legend to direct children only.

• Having a single child level is conceptually a better idea (why else would every other vendor do this), because it emphasizes the hierarchy much more clearly and can significantly reduce clutter...
• ... but some users like the current way of bunching them all together without any structure.
• So if we want to implement this, we will either need to implement it as:
  • A global ACP setting which limits the depth of the children in the legend (or no limit at all)
  • A setting that limits the level on a per-forum basis, configurable for each forum/category (or no limit at all)
  • A UCP setting so that users can set it themselves
• Show all subforums by default (same as current behavior)

http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=45585