Add some kind of callback to modify popup window generation

[hybridherbst]

Right now, the popup is only created, but not stored in any symbol as far as I can tell and so not possible to access from outside, e.g. to attach key listeners, drop handlers, and other nice web things.

• https://github.com/Looking-Glass/looking-glass-webxr/blob/fc5b610b860e0f15f70dff2c98c0c17ea58bc051/src/LookingGlassXRWebGLLayer.js#LL293C48-L293C48

Would be great if there was a way to provide a callback for popup creation to modify what that does and allow attaching other logic.

[BryanChrisBrown]

agreed on this, we can definitely expose that as a variable similar to the config setup.