[FEATURE] added service and manager for toolbar items for sidebar widgets

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [x] submit changes to the documentation

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | https://github.com/sulu-cmf/docs/blob/master/detail-specification/100-basic/DET-106-Widgets.md#widget-toolbar


__Requires:__
- https://github.com/sulu-cmf/SuluAdminBundle/pull/129

@michaelzangerle I would give this variable a more descriptive name like `widgetToolbarItemServices`