Older macOS cannot interpret the `CFBundleIconFiles` key so we cannot
rewrite the `CFBundleIconFile` entry without special consideration. I
opted to fix this by inverting the squircle logic. We always add both
the `CFBundleIconFile` and `CFBundleIconFiles` keys. The former is
necessary for at least macOS 10.13 and probably 10.12. The latter seems
to be ignored on those versions and overrides the former on newer
versions of macOS.
Inverting the logic also allows us to rely on the `toPlist` generator to
generate the XML syntax, which is a nice bonus.
- Convert icons to a single .icns file; and
- Provide an opt-out via X-macOS-Squircle in the desktop item to
override the squircle behavior when the source icons look bad when
converted automatically.
