Add API to capture screenshots of desktop/screen

[freakboy3742]

What is the problem or limitation you are having?

As an app developer, it is sometimes necessary to capture the current entire desktop state of a GUI 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 (e.g., an actual "screenshot" app)

Describe the solution you'd like

1. A cross-platform API to capture an image of the full screen.

2. As an example of the API in action, modify the testbed test harness so that when a 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

• See Add API to capture screenshots of windows. #2062 for a similar feature at the level of a Window.
• The API proposed in Added APIs for detecting multiple displays and setting windows on them. #1930 provides an entry point for accessing screens; based on that API, App.screens[n].as_image() would seem a logical API.
• 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 window_probe() and/or app_probe fixtures in 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).

[proneon267]

Since, #1930 is merged, I think this can be closed.

[freakboy3742]

Agreed - thanks for the housekeeping!