Write a warn if a URL is created from a internal GraphicObject
URL as this is now deprecated.

Extract the way to determine if a URL is internal to its own
function, so it can be used to check and rewrite places that use
it.

Change-Id: Iedf13a596eab839094ea98e41787865bf6783adf
Reviewed-on: https://gerrit.libreoffice.org/47495Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>