Provide a more extensive default bootstrap #2006

[Xzenergy]

PR Checklist:

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

[Xzenergy]

Thanks for the PR, but there's a couple of issues here.

Firstly, our contribution guide details how to set up a pull request so it has the best chance of passing CI. At present, your PR is failing some basic checks.

More importantly, the implementation isn't really solving the problem. You've added an "empty.toml"... which doesn't reflect the way any of the other bootstraps are defined. You've also made a lot of changes to the new and convert commands that only appears to mention the Toga and Empty templates... missing the other pre-existing templates. Lastly, you've added something to briefcase.toml... but the code you've added won't do anything.

A better approach for this would be to take an existing bootstrap, search for everywhere in the code that it is mentioned, and duplicate those references - i.e., create a "toga2" bootstrap and then rename that bootstrap "empty" - then use that bootstrap when "no GUI template" is specified (the current "None") option.

Thanks so much for the feedback, this is my first attempt at contributing anything to anywhere! Thanks for the contributions guide, I hadn't visited that yet. I'll do some reading and then try to re-tackle the problem. Thanks again!