Pick the backdrop by path, seeded with the wallpaper

[winebarrel]

The Desktop picture backdrop showed the wrong picture whenever the wallpaper was
a dynamic or video one. This replaces it with a checkbox and a path field.

Why the old one could not work

NSWorkspace.desktopImageURL(for:) predates WallpaperKit and only reports still
pictures. With the com.apple.NeptuneOneExtension provider set — Sonoma's dynamic
and video wallpapers — the wallpaper store holds no file at all:

provider: com.apple.NeptuneOneExtension
Files:    []

and the API answers with a placeholder,
/System/Library/CoreServices/DefaultDesktop.heic, plus an empty options
dictionary. The saver was drawing that placeholder faithfully. No public API
reports those wallpapers, so there is nothing to fix on the reading side.

What it does instead

There is one source for the backdrop now — a file — and the desktop's wallpaper is
only where its value starts. The path is filled in once, on first run, and is an
ordinary saved setting from then on: changing your wallpaper later leaves the saver
alone.

That removes the awkward middle case the old control had, where Desktop picture
silently followed an API that could not see the wallpaper. The placeholder can
still turn up as the seeded value, but now it is sitting in a field, visible and
correctable, rather than deciding the backdrop from behind the scenes.

Cats:        [ ^ v ]                              1
Size:        [---|------]                     64 pt
Speed:       [----|-----]                     1.00×
[x] Picture: [ …reServices/DefaultDesktop.heic ] [ Choose… ]
Grey level:  [|---------]                        0%
             [x] Paw prints
             [x] Scratch marks

Unchecking the box, or emptying the field, leaves the grey.

Bookmarks dropped

Storing a path rather than a bookmark drops the security-scoped bookmark
machinery added in the previous change. A typed path is not a user-selected file
in the sandbox's sense, but legacyScreenSaver.appex can read anywhere under /,
so the read succeeds either way and there is nothing left for a bookmark to buy.

Layout

The desktop's own scaling, clipping and fill colour still apply when the path
happens to name the very picture the desktop is showing, which is the usual case
given where the value comes from. Anything else is laid out as Fill Screen.

Verified

• Both schemes build and analyse; swiftlint --strict, swiftlint analyze --strict
  and swiftformat --lint are clean
• Seeding from an empty preference domain writes the wallpaper path — here, the
  DefaultDesktop.heic placeholder, which is the case this change exists for
• Offscreen renders for an explicit --picture, and for --grey
• The Options sheet layout, with the path truncated at the head so the file name
  stays readable

The Choose… panel needs a GUI to exercise, so it is unverified, as is the whole
saver in System Settings on a real machine.