It seems the manager -> assistant -> employees ordering is not part of
the file format. The order is stored twice in the file: the hierRoot
algorithm has 3 layout nodes as a children, and also the data model has
an order of the presentation nodes: both describe that employees go
before assistant nodes.

In contrast to that, PowerPoint orders XML_asst nodes before XML_node
ones, so teach the hierRoot algorithm about this.

This requires tracking the data model node type for each in-diagram
drawingML shape, so that layout can determine if a hierRoot algorithm
children has an assistant node or not.

Change-Id: Ib81f3666fb092ed3b036d830f69ba7e1b94f8331
Reviewed-on: https://gerrit.libreoffice.org/66048Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins