Rotary Slider

[mitchmindtree]

Like this:

More compact than regular Slider, useful for sliding ranged rotations etc.

[clicketyclack]

This seems like an interesting challenge. As far as I can tell, it would require adding a wrapper for piston CircleArc.

I assume adding a primitive would need adds in both render.rs and a new struct as
circlearc.rs here.

No idea about how to get values from the resulting widget though. Only been thinking about the rendering part so far.