oox smartart: snake algo: start parsing relative width constraints

oox smartart: snake algo: start parsing relative width constraints

This is needed, but not enough to have the correct width for each shape
and spacing item on the path of the snake. Currently we give the same
width for all children, while the shapes typically have a larger width
than the spacings.

Change-Id: I4e2638ea5b566c21cb1cf503a8cf2e5d35256e3c
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/103594
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins