[Improvement] Add missing postAddAssetTree event and adding parentId to details

[mattamon]

For adding new elements in e.g. object and document tree there is a postAddTree event.
This event is missing for the assets.

For easier access also to the parent the id of the parent should be in the details.

[github-actions]

Thanks a lot for reporting the issue. We did not consider the issue as "Pimcore:Priority", "Pimcore:ToDo" or "Pimcore:Backlog", so we're not going to work on that anytime soon. Please create a pull request to fix the issue if this is a bug report. We'll then review it as quickly as possible. If you're interested in contributing a feature, please contact us first here before creating a pull request. We'll then decide whether we'd accept it or not. Thanks for your understanding.