[BUGFIX] CSS: Default vertical spacing for blocks

Forms, tables, texts need a clear structure and obvious
visual distinction. Our Backend currently does not
deliver strong defaults for extensions.

Many modules already work around this lack of structure
by adding extra spacing and horizontal rules:

- DocumentTemplate::spacer(x-Pixel)
- DocumentTemplate::divider()

This patch brings vertical spacing based on CSS for:

- p
- ul / ol
- dl
- blockquote

Change-Id: I93a1a43b21d6e56db822558c6d4be0103097ed98
Resolves: #48540
Releases: 6.2
Reviewed-on: https://review.typo3.org/21050
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Merge remote-tracking branch 'origin/master' into 7elix-TYPO3.CMS-master

Conflicts:
	typo3/sysext/t3skin/extjs/xtheme-t3skin.css
	typo3/sysext/t3skin/stylesheets/structure/main_content.css

Change-Id: I6c7fedd96010dd9f96b4fb5634f2d46a7a5a6668