Means for watching categories

Rationale
On big wikis patrol of new pages is not filterable by topic. This reduces the contact of WikiProjects with newly submitted articles. Such contact would be beneficial to the articles' early life-cycle (and hence to the ones who authored it).

Specification
The idea is to have means of monitoring articles that were recently added to a category.

To make this meaningful, it is necessary to also introduce an activity of picking up newly created articles and categorizing them. Articles that do not get categorized get displayed on a special "uncategorized" list.

The output may have such features as:
 * Recursive. On big wikis one can not possibly remember all subcat names.
 * Multiple. It may be needed to monitor multiple categories.
 * Timed. It should be interesting to view articles that were created within last 24 hours.

Possible entry points
While implementing this feature, different entry points are possible. It is necessary to pick ones that are both intuitive, and do not require to re-invent the wheel. (There are some nice things Special:NewPages and all sorts of similar pages already do, such as showing author, timestamp, edit summary, tags, and coloring by page review status.)
 * Click a link at a category (takes to an external tool at WMLabs) [+the external tool].
 * Click a link at a category (takes to an RSS feed provided by an extension, such as Special:Newsfeed (aka Extension:GoogleNewsSitemap)) [+option for the extension to go recursively]
 * Watch a category for recent additions (appear in watchlist) [+new watchlist hook]
 * Visit Special:RecentChangesLinked (appears there) [+option to go recursively; +option monitor multiple categories there]

- Extension:DynamicPageList (Wikimedia) exists, but it is rather slow due to the intersection stuff. Making something similar that relies on Wikidata might help....