[TASK] Simplify Font-Size inheritance (CSS)

Font-sizes are defined in t3skin at various places multiple times.
In order to reduce complexity font-size & line-height should be
defined centrally and inherited by containers and elements.

Removes few definitions to unify font-sizes and line-heights.
Also fixed ExtJS CSS theme.

Change-Id: I6493aca9c2dd0fe36c19a65b1e6076b6a181419a
Resolves: #52497
Related: #52501
Related: #49592
Releases: 6.2
Reviewed-on: https://review.typo3.org/24299
Reviewed-by: Marcin Sągol
Tested-by: Marcin Sągol
Reviewed-by: Stefan Neufeind
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny