Fix Categories conversion to Doctrine 2

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | yes
| Tests pass?   | na
| Fixed tickets | 
| Refs tickets  | #677, #680, #740
| License       | MIT
| Doc PR        |

Working
 - [x] Installs
 - [x] creation/editing of category
 - [x] creation/editing of category attribute
 - [x] creation/editing or category registry entry
 - [x] installation of PostCalendar (uses `CategoryUtil::createCategory()` & `CategoryRegistryUtil::insertEntry()`)
 - [x] installation of News (uses `new Categories_DBObject_Category()` and `new Categories_DBObject_Registry()`)
 - [x] Upgrade routine for `categories_attributes` (new) from whatever happened before

Todo
  - [x] Documentation.

This is a stub ticket for collaboration.

This PR introduces the D2 refactoring again freshly so it can be worked on collaboratively - meaning I need some new eyes to take this patch to completion.
Refs #677 #680