[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           |
tehcnically it makes no difference. Historically I have always done ordering in steps of 10 to make manual reordering a possiblity. You can't put anything between 1 and 2, and we shouldn't have to as the nodes are reindexed automatically, but it does no harm and your never know.