[FEATURE] Bring tree icons in line-height

Adds taller tree icons to EXT:t3skin and put them into a new
sprite category "treeline".

This allows to use these icons without breaks in line-height of 1.5em.

Icons are 30px tall due to:

- line height: 18px = 12px * 1.5em
- Padding top+bottom: 12px = 6px * 2

Change-Id: I8a295dbf55b6847e89250c9c221259386aa362e6
Resolves: #52599
Related: #52501
Related: #49592
Releases: 6.2
Reviewed-on: https://review.typo3.org/24474
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny
Reviewed-by: Felix Kopp
Tested-by: Felix Kopp
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn