The "Vertical Box List" preset in PowerPoint contains two linear
layouts, the inner one nested in the outer one. All the constraints are
stated on the parent layout. This commit doesn't actually read those
constraints yet, but adds infrastructure to look up parents of layout
nodes and also to get the constraints of a layout node.

No functional changes intended.

Change-Id: If844d17a90f8f180a9d21245249a2744bea32b31
Reviewed-on: https://gerrit.libreoffice.org/62367Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins