Add categorylinks table to separately list category relationships. Actual

Add categorylinks table to separately list category relationships. Actual
_page_ links to category pages like [[:category:Some cat]] will stay in
links/brokenlinks and not be listed in the category page list anymore.

A link can optionally specify a sort key like this: [[category:cat|Sort me]].
The page will then be sorted in category lists according to the given text
instead of the page name; thus categories may be sorted arbitrarily, by
last name or whatever.

There is also a timestamp field included on gwicke's request; this is not
used yet. Also it will currently be updated at every edit of the page,
this can be changed by implementing differential updating for the cat links.

TODO:
* Make sure that automatic sort keys are updated on page rename.
* Make sure cateory pages get cache-invalidated and purged
* Use proper text sorting instead of raw binary sort
* Allow specification of the display style on category pages: comma list
  or bullet list or number list; by sortkey or by timestamp; etc