[FEATURE] Added node order property

The node order property is automatically calculated on a new node. When
a node is moved the node order is updated on all of it and all of its
sibling nodes.

This has been implemented with an event subscriber.

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | prerequisite for fixing https://github.com/sulu-cmf/sulu-standard/issues/247
| BC Breaks     | NO
| Doc           |
Maybe call this one `NODE_POST_SAVE` if the other event is called `NODE_PRE_SAVE`?