[TASK] EXT: t3skin: Extract form styling into own file

File main_content.css currently includes styles definitions
for <form> and corresponding fields.

Extract CSS styling definitions from main_content.css into an
own file "main_form.css" (for /structure + /visual directory).

Change-Id: I77fb16fb32d28a1e946e5f6093ee4389289e0ca1
Resolves: #48783
Releases: 6.2
Reviewed-on: https://review.typo3.org/21196
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Markus Klein
Tested-by: Markus Klein