Generalize template for GUI framework plugins

[rmartin16]

Changes

• Generalizes the template to support arbitrary GUI frameworks
• RE: Add support for third-party GUI framework plugins briefcase#1524

BRIEFCASE_REPO: https://github.com/rmartin16/briefcase
BRIEFCASE_REF: run-app-in-ci-testing

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]

Mostly straightforward stuff, subject to the changes in the matching Briefcase PR. Only major issue is about testing strategy; details inline.

[freakboy3742]

Slightly odd whitespace usage here...

[rmartin16]

I was inspired by the way that these constraints are interpreted by the packaging infrastructure such that the operator is really a part of the version.

>>> from packaging.requirements import Requirement
>>> str(Requirement("toga == 0.4.0").specifier)
'==0.4.0'
>>> str(Requirement("toga >= 0.4.0, < 1").specifier)
'<1,>=0.4.0'

But I should make them consistent with the rest of our dependency specifications.

[freakboy3742]

I'm not sure this test is validating what it once did. Previously, it was a check that the code generated by the template was actually valid, and passes flake8; however, now, it's doing those checks mostly on values that have been provided by the test itself. All the interesting logic that needs validation is in the Briefcase repo now.

A briefcase new --noinput && briefcase build check is probably called for to ensure the template is actually valid and results in compilable content; but beyond that, the place we need to test this is in Briefcase.

[rmartin16]

Yeah; that's something I've been struggling with for these changes. I have a similar comment in beeware/briefcase#1524.

I think the tests here are still relevant as-is, though....since the template itself does have some control over creating a valid project....as I've learned by having to seemingly endlessly tweak the jinja2 templating to play nicely.

We can consider the broader testing strategy in beeware/briefcase#1524.

[freakboy3742]

One error found in the template, but otherwise this looks good to go once the repo reference is updated.

[freakboy3742]

Should this be iOS, not macOS?

[rmartin16]

Yep; thought I got em all. Thanks.

[freakboy3742]

Marking as approved, conditional on changes that are needed as part of the beeware/briefcase#1524 landing strategy

[freakboy3742]

Approved for final ship