remove OSB namespace

Marc Jeanmouginrequested to merge
marcjeanmougin/inkscape:no-osb into master

Replaces osb:paint with inkscape:swatch attribute.

See https://gitlab.com/inkscape/inbox/-/issues/2904 for "why"

Most of the commit removes it in share/ svg files; file.h / file-update.cpp updates it in old documents, and src/xml/* should be refactored into c++

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