The file Assets.car is a precompiled asset catalogue, generated by the
xcode build (which knows how to do this from the raw Assets.xcassets).

It's not clear how to do this the right way with cmake, but also the
compiled asset catalogue should never change; therefore I am just including
it here with the macOS icon "sources".

If the app icon or document icon should ever change, this can be regenerated
by building the bundle in Xcode and copying the resulting Assets.car out
of the build products folder.

For reference, the command-line way to compile Assets.car seems to be
something along the lines of:

actool --compile destination_folder --app-icon AppIcon \
  --platform macosx --minimum-deployment-target 10.13 \
  --output-partial-info-plist path_to_output_plist \
  path_to_Assets.xcassets

