#61 absolute paths in XML storage

[Mark Lentczner]

When a tree is written to XML storage, and the tree has paths that are
absolute in form, but refer into the tree, they should be written out as
relative, but with a flag to absolutify them on read in.

Absolute paths that are actually tree relative can occur in three
places: prototype links, path objects and links.

Prototype links are always absolute (as of 2005-06-03), as so don't
need the flag, but should just be treated as if it were true.

Proposals for how to write this flag are:

<path ref="../../ball" fixup="true" />

<path ref="../../ball" style="abs" />

<path abs="../../ball" />

<path from-relative="../../ball" />