Drop the grey level, and settle on black

[winebarrel]

A slider for how grey the empty backdrop is earns its row in the sheet only if
someone wants a grey that is not black. The cat is black ink on a white body,
which reads best on black; every other value on that slider was a way to make it
read worse. With no picture to draw, the backdrop is now black, and the slider is
gone.

Options

Cats:        [ ^ v ]                       1
Size:        [---|------]              64 pt
Speed:       [----|-----]              1.00×
[x] Picture: [ Path to a picture     ] [ Choose… ]
             [x] Paw prints
             [x] Scratch marks

What went

background, backgroundRange, its stored key, its entry in the registered
defaults, and its row in the sheet.

The litter ink still reads the backdrop rather than assuming one: over a picture
it is measured as before, and over the plain black it comes out light, which is
where a measured brightness of zero leads anyway.

The preview app's --grey becomes --no-picture, since there is no grey left to
ask for.

Verified

Both schemes build and analyse; swiftlint --strict, swiftlint analyze --strict
and swiftformat --lint are clean. Rendered the Options sheet (row gone, layout
still square) and a frame with no picture (black, cats and prints legible on it).

Note

This touches the Options table in the README, which #8 also rewrites. Whichever
lands second will want a hand with the merge.