Distance and Angle Path Anchors

[johannes-wolf]

This PR adds path anchors for fixed and relative distances to most elements.
Closed elements (circle, rect, arc) can also handle angle anchors.

This means:
(name: "element", anchor: <number, ratio, angle, string>)
can be used to get dynamic anchors on element paths.

• Arc PIE mode line strip has been fixed and is now the last element, after the arc so that all arc paths start at arc-start.
• All paths got start, mid and end anchors that point to 0%, 50% and 100% of the path.