[BUGFIX] FormEngine paddings, margins, colors, borders

Fixes several layout issues with the FormEngine form:

Adds space between checkbox+label.
Forms element browser link as default button.
Form field header will be a headline h4.
Repairs outer padding and inner padding and borders.

Resolves: #62466
Releases: master
Change-Id: I0dcf1cd2206c0cee63a8353516a3b41b94fd670b
Reviewed-on: http://review.typo3.org/33520
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>