handle attribute `href` according to SVG2 spec

What does the merge request do?

handle attribute href according to SVG2 spec

defaulting to xlink:href for compatibility

Relates: inbox#620 (closed) Relates: inkscape!4596 (merged)

Implementation notes

getter is reading href and falls back to xlink:href setter is updating href (if present) and defaults to xlink:href

Checklist

• Add unit tests (if applicable)
• Changes to inkex/ are well documented
• Clean merge request history