Add "New with…" to File menu

[jeremy-w]

Select multiple images. Confirm. Each one spawns a new image slicer document for that image.

This will work best if we get the location remembering and save location defaulting working first #4.

Might want a Save All… after that to stamp them all to disk in default locations.

[jeremy-w]

Turns out that dropping the images on the app icon already works. Like magic. I forgot I implemented that! So, no need for "New with…"

Save All could still be handy, though, but I'd want to do an uncontested save. And probably not really worth it, honestly. I can save them as I mark up, or drop again.