[CLEANUP] Code cleanup in ext:core/Collection

* Use short class name where possible
* Correct curly bracket at end of defining the interface
* Create getDatabaseConnection method

Resolves: #62704
Releases: master
Change-Id: I2f52a045a34aa188e79f9b729e3df34a27bf9220
Reviewed-on: http://review.typo3.org/33800
Reviewed-by: Stefan Froemken <froemken@gmail.com>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>