[TASK] Clean up form system extension

* remove declare UTF-8 prologue
* remove author from PHPdoc of methods
* change visibility of methods and properties from private to protected

Change-Id: I3cf8a6d22dc524c028de3ae22888ee4d414d0064
Resolves: #28656
Reviewed-on: http://review.typo3.org/4000
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader