Gui: on the frontend, the control callback does not get the context it set when it was registered

[giuliomoro]

e.g.:

let controlCallback = (data) => {
     console.log(this);
}
Bela.control.registerCallback("controlCallback", controlCallback, { val: 1, otherval: 2});

prints

Window [window: Window(0), self: Window(0), document: document, name: '', location: Location, …]

I don't see any obvious issue in BelaControl.js:

		Object.values(this.callbacks).forEach( c  => c.function.call(c.object, parsedData) );

seems to be doing what it's meant to, but call() doesn't seem to set c.object as this as it should.