[TASK] Migrate FormEngine Checkboxes HTML to twbs

Change CheckboxElement HTML towards twitter bootstrap
and refactor the class a bit for better readability
along the way.

Resolves: #62975
Related: #62973
Releases: master
Change-Id: Iacc7bb77fb604178a647e3d3e79eaa1677738c20
Reviewed-on: http://review.typo3.org/34199
Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>