* both width and height of children and space is taken from constraints
* better handling of space between children (not lost in some cases)
* children centered in the other axis

Change-Id: I25b8360790de0292b2b5c313dfa55e58dc042193
Reviewed-on: https://gerrit.libreoffice.org/73201
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>