Add a post-generate extension point for bootstraps.

[freakboy3742]

In the process of working on #1288, I discovered another extension point that is required on bootstraps - a "post generation" step that is invoked after the template has been generated. This is required because the Briefcase template has a fixed set of files; if you need to write an additional file (for example, to write an index.html for an embedded web server), there's not currently any way to do that with the bootstrap.

This requires a slight reorganisation to how the bootstrap is created, so that the bootstrap instance can be retained until after the template is generated. However, by doing this, it becomes helpful for there to always be a bootstrap... which also provides an opportunity to address #2006.

Makes 2 other small changes to the Console bootstrap:

1. Adds some additional documentation to the code generated by the Console bootstrap to match that generated by new Empty bootstrap
2. Renames the override variable for specifying a console app in the convert command. This hasn't been in an official release, so there's no backwards compatibility concerns; but specifying "-Q console_app=GUI` doesn't make any sense from a UX point of view.

Refs #1288
Fixes #1899
Fixes #2006

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

[rmartin16]

lgtm w/ some minor comments.

This also addresses #1899. And it may make #1898 even more difficult to address....although, as you said, a more general approach with PEP-621 will likely reshapre that whole conversation.

[rmartin16]

Suggested change

	"""Code to run after the template has been generated.
	"""Runs after the template has been generated.

[rmartin16]

I was a bit caught off-guard when this didn't work:

briefcase convert -Q app_type=console

-- Interface Style -----------------------------------------------------------

Using override value 'console'

Invalid value; console
Invalid override value 'console' for Interface Style, using user-provided value.

Is this a GUI application or a console application?

  1) GUI
  2) Console

Interface Style [1]:

Maybe better to make these comparisons case-insensitive....

[freakboy3742]

This also addresses #1899.

Good catch - I've added a changenote and a "Fixes" annotation in the PR body.

And it may make #1898 even more difficult to address....although, as you said, a more general approach with PEP-621 will likely reshapre that whole conversation.

Yeah - I think the real fix for #1898 is to stop using keys that are already covered by PEP 621, which is a much bigger project. I'm not sure this makes #1898 substantially worse though - the only project-wide keys it's added are ones that are cumulative; and the platform-specific keys are all either cumulative, or not covered by PEP 621.