fix: is_visible()

What does the merge request do?

fix: is_visible() to answer consistently with modern UserAgents (i.e. Firefox)

Implementation notes

• rely on cascaded_style() to include CSS and presentation attributes
• include visibility attribute with check for inherit
• include ancestors

Summary for release notes

revisited is_visible() to answer if element is visible in UserAgents (compliant with rendering in browser)

Checklist

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