Preserve `imageOpacity` when assigning to `Wheel.items`

[evhan]

The imageOpacity property wasn't previously passed through when initializing Item objects, so it would be ignored.

One can work around this by setting imageOpacity manually, as in the following example, but it seems like this property is simply missing from the setter and should be included with the rest of the fields.

var wheel = new Wheel(container, { items })

for (const i in items) {
  wheel.items[i].imageOpacity = items[i].imageOpacity
}

[CrazyTim]

Thanks @evhan!