CustomPrompt needs a callback for the onChange method

[neocybereth]

I have a CustomPrompt with some images I've placed within it.

Maybe b/c they're not children of the Custom Prompt, whenever I close it the images still stick around.

The custom prompt needs an onClose function so that some arbitrary functions can run before the native this.hide() is run to close the prompt.

Currently the best I can do is CustomPrompt.background.onChange = [func here] but this is run AFTER this.hide() so there's a brief flash when the images are removed.