Add parameter that allows to add image to the Rate us pop up

[slvlntsv]

Adding image allows more creative custom pop-ups, which may attract more attention:

[Skyost]

Hey,

Thanks for the PR. The contentBuilder should be what is passed to the dialog main widget, as is. It's because it allows developers to be master of their dialog 🙂

Also, I think that contentBuilder is a sufficient feature for advanced customization of the dialog.