improve arc support

Circular progress bars like http://www.cssscript.com/wp-content/uploads/2017/01/Circular-Progress-Bar-With-Plain-HTML-CSS.png are more difficult than they should be to create/animate via PathArc. Consider adding a declarative arc element with an API based on e.g. angle (like that of QPainter or https://www.w3schools.com/tags/canvas_arc.asp) to make this common use case easier.