Improve screenshot capture function

[lassoan]

Currently self.tutorial.nextScreenshot() method is called to capture a screenshot. The problem with this is that the generated output cannot be easily associated with screenshot used in the documentation file. => the method should take in identifier (e.g., "012-part1-after-mri-loading")

Currently, each window is captured as a separate image. It means that each slice view controller floating widgets are captured as separate windows instead of part of the application screen layout. => change screen capture method to capture a single screenshot (capture each window and merge the images into a single image). In the future we could implement support for capturing only certain window (e.g., identified by specifying a widget that is inside that window), but it is not needed for now.

Method API would be:

def captureScreenshot(id: str)
    """Capture screenshot of the application window and all popup windows of the application.
    id: string that uniquely identifies the screenshot in the documentation, recommended to start with a 3-digit numeric code,
        followed by a short text. For example: `105-part2-clipping`
    """

[lassoan]

Please update the tutorial at https://github.com/SlicerLatinAmerica/TestSlicerTutorials/blob/main/Tutorials/FourMinuteTutorial/FourMinuteTutorial.py (and remove the test tutorial from this repository). We need to maintain tutorials in repository from this one, as the tutorials may come with large binary files (screenshots, maybe some smaller sample data sets, etc.) that we don't want to pollute this repository with. Also, tutorial authors would be different from the extension developers, so it is better if they use separate repositories.