Refactor: Move update path logic to Node model

- Add 'plugin' key to _updateAllNodesPath()
- Add test for Node::updateAllNodesPaths
Refactor: Extract method Node::_findPromoted
Add quick tests for beforeFind / beforeSave callbacks.
Refactor: Extract method Node::_getType()
Refactor: Move processAction logic to Node model.
Merge branch '1.5-Nodes' into 1.5

Refactor Nodes plugin: moving logic to model.
Closes #321