feat: also accept string listing points i.e. `path="1,2 3,4 5,6"`

What does the merge request do?

feature: add Polyline.new(points) and Polygon.new(points) where points can be one of

• points=[(1,2), (3,4), (5,6)]
• points="M1,2 L3,4 L5,6"
• an inkex.Path
• points="1,2 3,4 5,6"

Implementation notes

• points is set via set_path() method
• Polygon inherits from Polyline

Summary for release notes

add Polyline.new(points) and Polygon.new(points)

Checklist

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