[TASK] Set base font-size and adjust headline sizes

Set body base font-size system-wide. Adjust base font size and
increase by 1 Pixel. Inherit base font size for headline based
on relative sizes. Give headlines better visual hierarchy.

Change-Id: Ie7f7acad607aec937d8157f94762c16260ad1b9c
Resolves: #52501
Releases: 6.2
Reviewed-on: https://review.typo3.org/24307
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn