ArcballControls: Allow setting the gizmos radius factor

[Tirzono]

Description

I would like to be able to set the gizmos radius factor. Currently the only way to do this is to extend the ArcballControls and overwrite the calculateTbRadius function, but I think it would be cleaner to have a way of setting it as a property.

@danielefornari let me know if you are happy with this (thanks for your great work on ArcballControls!) :)

[Mugen87]

@Tirzono It's not necessary to do it in this PR but it would be great if you add both new properties to the documentation page of ArcBallControls: https://threejs.org/docs/index.html#examples/en/controls/ArcballControls

[Tirzono]

That's not a problem, let me know what you think. I'll also add it to the other PR.

[danielefornari]

Indeed is a nice feature to have! Beside radiusFactor, i would also add a setTbRadius(float) method which updates the radius property, redraw gizmos and send a changeEvent to visualize changes in real time.

[Tirzono]

Sounds like a good idea. I am not sure if I have time for that to add before the weekend, but if not then I'll add that after the weekend.

[Tirzono]

I've added the setTbRadius function. Let me know what you think or if it needs any more changes.

[mrdoob]

Thanks!