Add API to capture screenshots of windows.

[freakboy3742]

What is the problem or limitation you are having?

As an app developer, it is sometimes necessary to capture the current state of a GUI window as an image.

The most notable use case is in CI testing, where the screen isn't visible, and test failures are most obvious in their visual presentation; however, there may be some other uses.

Describe the solution you'd like

1. A new cross-platform Window.as_image() -> toga.Image API.

2. As an example of the API in action, modify the testbed test harness so that when a widget test fails, a screenshot of the screen is captured and written to the cache directory.

Describe alternatives you've considered

Do nothing. Screenshots aren't essential; they're a nice-to-have feature.

Additional context

• The Canvas widget already has an as_image() API, demonstrating how to save a single widget as an image. The Window screenshot API should be almost identical, but applied to the root content widget.
• The Canvas GUI tests already have tests that write the as_image() representation to disk if the canvas hasn't rendered correctly.
• It should be possible to catch a failed test in the teardown of the widget_probe() fixture of testbed; if an AssertionError is raised during the yield, that will be a test failure; if this occurs, take a screenshot of the window/screen, write to the app cache folder, and re-raise the assertion (so that it is picked up by Pytest).

[dr-gavitron]

Started implementing it. Work done ✅ for windows and Linux. Will submit a PR when completed for all platforms.

Add pillow features in this: as_image will return toga.Image when nothing will be specified. And will return Pillow Object when PIL.Image is specified as format.

[freakboy3742]

@dr-gavitron If you read back to the tickets that are linked to this one, there's already an implementation of this on all platforms as part of #2103.