svtools: one shared policy for following URLs out of document content

Each module that follows a URL named by document content -- linked graphics,
background images, instance data, media -- decided

svtools: one shared policy for following URLs out of document content

Each module that follows a URL named by document content -- linked graphics,
background images, instance data, media -- decided for itself whether to
check anything, and the check that does exist was written out inline at the
call site.

Add svtools/linkpolicy.hxx as the one place that decides, and apply it at the
load sites in editeng, forms, sfx2, sw, avmedia and svgio. A relative
reference and a file on this machine load as before; anything else asks the
running document, which is the question the LinkManager already asks.

Two places on those paths assumed success and no longer do: a reference that
cannot be resolved, and a graphic that is not there.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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