Fix spacing between title and sections
Fix spacing between title and sections

This commits removes complex selectors and also removes the use of
padding/min-height to be able to use the margin merge.

All items are seperated by 2rem. Title gets an extra 2rem top margin,
for a total of 4rem between a section and a title. Of course, the first
title only gets the required amount of margin to be able to have a 2rem
gap.

Empty elements get 1rem of top margin. Next empty element will get 0
bottom margin, to keep a 1rem space between empty elements.

Re #2698
Re https://github.com/symphonycms/symphony-2/pull/2698#issuecomment-309451014

symphonycms/symphony-2,michael-e/symphony-2,symphonycms/symphony-2,jensscherbl/symphony,jensscherbl/symphony,bauhouse/sym-todo-list,michael-e/symphony-2,bauhouse/sym-todo-list