group bounding_box: exclude invisible children

What does the merge request do?

• For the bounding_box of group elements exclude invisible children to comply with Inkscapes bounding_box computation
• The shape_box of group elements returns the shape_box value of its children (not bounding_box)
• Add tests for clipped paths (shape_box and bounding_box)
• Add tests for invisible path (bounding_box)
• is_visible works now also for unrooted fragments

Summary for release notes

group.bounding_box() excludes invisible objects group.shape_box() returns a fully unclipped result for all child objects

Checklist

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