[TASK] Introduce relative line-heights

The line-height for the backend was fixed to 14px.
But line-heights should be relative to the font-size.

With this standard line-heights are 1.5 x font-size.

Change-Id: I2c098141b4905b1ac7bf756266e77dd2078ea6f2
Resolves: #51000
Releases: 6.2
Reviewed-on: https://review.typo3.org/23021
Reviewed-by: Ingo Pfennigstorf
Tested-by: Ingo Pfennigstorf
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters