API Added SiteTreeExtension::updateRelativeLink

This API update finally gives extensions the ability to interact with SiteTree urls, so modules such as Translatable, and other SEO-based ones, can do so without requiring hard coding or requiring users to override RelativeLink.

I've refactored out the Translatable hook here and moved it to another pull request for the translatable module at https://github.com/silverstripe/silverstripe-translatable/pull/109

I also have two other modules in mind waiting on this functionality. :)