add rules integration

I'm working on integrating CCK with rules. This includes a condition to check values of fields, an action to populate field values and two actions to load referenced nodes / users.

Note that I wanted to write the "populate a field" action as core action, however I found no way to do a multistep form there, so I gave that up and concentrated on a rules compatible action.

status of the patch
* user/node reference actions work fine
* populate field action basically works, but needs polishing
* the condition will be added once the action as done, as both can share 99% of the code.

Known bug: I've got the AHAH functionality working in the populate a field action configuration form, but the form dies due to the activation of form caching. Forms without AHAH work fine. (It let me run over http://drupal.org/node/299672 though :)

I'm posting this although it's not ready yet, as I'd like to get feedback from other devs:

* I'd like to get the integration into CCK instead of letting rules provide it - as it's the way it should be: rules provides hooks and CCK makes use of them. The include files are automatically conditionally included by rules anyway and of course I'd help maintaining it.

* The populate a field action is similar to the default value stuff in the field configuration screen. It embeds the fields form and also allows using PHP code. So I think reusing the validation routine for it would make sense. For that I'd like to factor that part out of _content_admin_field_validate(), so it can be reused.

* The content.rules.inc is sitting in the module main directory, as unfortunately this is required that rules can find it. Is this a problem?

* I've split the integration into three parts, so that the integration of the user/node reference fields goes into their modules. Is this ok, or should I combine this into one include conditionally adding in the actions for user/node reference once their modules are activated?

Current patch attached as well as a screenshot, showing the "populate a field" configuration form with an example field.