2 Feature Requests: Let user decide the size of the circle / use existing canvas

[orangecoding]

Hey,

• if I chose to display a radial visualization, I'd like to be able to tell how big the circle actually is
• I'd like to connect audioMotion to a canvas, which I define by myself rather than letting your lib creating it

I think those 2 optionals would make sense to be added, wdyt?

Thanks Chriz