Add Init Image/Edit Image specific prompt and metadata for edit prompt

[aimerib]

Adds a prompt box for init image params, specifically for using with edit image, but would work with init image as well. When a value is present in this prompt, the main prompt is ignored.

This will also add editprompt to the list of metadata.

This change is useful for when you have a long involved prompt as your main positive prompt, and you want to edit an image, but doesn't want to lose your prompt. This gives users a more ergonomic experience than both having a separate image2image tab AND having to replace your main prompt for every edit.

I suppose that this is sort of redundant if you are only doing img2img starting from a random picture dragged to the UI, but it is really useful when editing a generated image.

This was a pretty simple change, mostly to address https://github.com/mcmonkeyprojects/SwarmUI/issues/303 and my own needs. I'm more than open to discussing improvements or other approaches to this problem. I tested locally, and it seemed to work really well.