Derive new app wizard properties from cookiecutter metadata

`briefcase new` runs a wizard to roll out a new application.

It asks a series of questions; however, the answers to those questions are hard-coded. As a result, if the template adds new options for licenses or GUI frameworks, those options won't be picked up.

The `briefcase new` wizard should be modified to read the question metadata from `cookiecutter.json`, and use the response lists in the template as candidate responses.

Related to this: the cookiecutter template should be updated *before* the questions are asked (otherwise, the list of available options may not be updated until *after* you've run through the wizard).