Correct the CFBundleIconFile entry

[freakboy3742]

The CFBundleIconFile entry didn't include the filename extension; as a result, trying to load that file failed.

Refs beeware/toga#2558.

Requires a back port to the 0.3.18 template branch.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Turns out this isn't the fix like I first thought. When Xcode builds the project, the Info.plist file is processed, and no matter how it's passed in, the .icns suffix is stripped. This is because macOS APIs load the icon by asking the bundle for the CFBundleIconFile resource with a file type of .icns, rather than just passing in a full filename for the resource.