Add a base class for doing form alters in the early installer

Problem/Motivation

The early installer (before the database exists) no longer invokes form alter hooks in the profile. This ability was a casualty of the OO-based hook system. Honestly, it probably only ever worked by coincidence anyway.

So the way to alter an early installer form now is to decorate it. That works, and Drupal CMS is using it, but it involves a pile of boilerplate. Recipe Installer Kit could add a base class to handle the boilerplate.