[TASK] Code cleanup: Clipboard.php

backend/Classes/Clipboard/Clipboard.php
To get it "green" in PhpStorm several changes were made:

- set correct parameter type in PhpDoc comment
- set correct return type in PhpDoc comment
- add missing return values
- add getter for LanguageService
- add getter for BackendUser
- add getter for DatabaseConnection

Resolves: #65428
Releases: master
Change-Id: I3079a845242239325616b682683cc79da36ee4f8
Reviewed-on: http://review.typo3.org/37402


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>