Write a BackLinks Plugin

We need to write a Backlinks plugin. This plugin is important for the following reasons:

1/ It is a major "Wiki" feature expected by Wiki addicts. Backlinks are often used as a way to categorize data in Wikis.

2/ The backlinks can be used to update links when a page is renamed.

The backlinks plugin should be architectured the following way:

1/ The plugin is notified on each update of a page
2/ The page is parsed and outgoing links are extracted (internal and external)
3/ Outgoing links are stored in a specific table in the database
4/ An API should allow querying the backlinks database (internal or external)

An administration feature should allow to recreate the full backlinks database from scratch.
Backlinks should be shown by default in the toolbar next to referers. Number of incoming clicks can also be shown (extracted from the statistics module)