Event for original isPressed flag

I needed access to original `isPressed` boolean in order to fly around the scene with the mouse press. So I have to add this in order to expose it.

```
if ( button.isPressed && isPrimary ) {
    controller.dispatchEvent({ type: 'primary pressed' })
}
```

I don't quite understand why it was not exposed original. It is such a basic thing to have.

I can make an PR for that if necessary.