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

__Tasks:__

- [ ] test coverage
- [ ] gather feedback for my changes
- [ ] 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