Add support for Apple watchOS

Now that the Python iOS support libraries support watchOS, Briefcase should also support that platform.

This will involve:
- Producing a cookiecutter template project (analogous to [Python-iOS-template](https://github.com/pybee/Python-iOS-template))
- Adding a new command to Briefcase to support tvOS. The implementation of this command will be almost identical to iOS, with names and images changed.