Dummy backend: implement Documents/Desktop/Pictures stubs; print docu…

[bekarys2003]

…ments in simple test app

This patch adds implementations for get_documents_path, get_desktop_path, and get_pictures_path in the Dummy backend so that app.paths.documents, app.paths.desktop, and app.paths.pictures resolve to the user’s ~/Documents, ~/Desktop, and ~/Pictures directories, respectively. It also updates the simple testbed app to print out the documents path (and optionally desktop/pictures) so that the existing user‐space path integration tests pass in non‐interactive mode.

Fixes #3551

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]

Thanks for the PR; however, as CI is showing, there's a lot of problems here.

First is that you've added an implementation for the dummy backend, and tests, but you haven't added the public interface layer (in the core module). It's the core module that exposes paths.* as something that can be accessed.

More importantly - adding this to Dummy is an important first step, but we're also going to need implementations for macOS, Windows and Linux - and exception-raising versions for other platforms. We're unlikely to accept a PR that adds an API, but then only implements in on the test platform.

[freakboy3742]

A couple of comments here:

• This should be 3551.feature.rst file - it's a new feature, not a miscellanous update; and it's RST content
• It should be phrased in the form of a release note:

Suggested change

	Implement Documents, Desktop & Pictures paths in the Dummy backend and update the simple test app to print them.
	App paths now expose locations for Documents, Desktop & Pictures folders.