Allow notification of only published or only unpublished nodes.

One of the main ways rules notifications were used was for notifying administrators when content was submitted that needed to be approved (unpublished content). There are also occasions where if you are sending notifications of new content to a non-user list, you would only want to include published content notifications.

I've written a patch to allow only notifying of published or only unpublished content. (Will be attached to subsequent comment.)

This patch also replaces EntityInterface with the more specific NodeInterface since this is only expected to run on content, and some functions are specific to node entities, not all generic entities.

See https://api.drupal.org/comment/62836#comment-62836 for reasoning.