feat: add defaults screencapture show-thumbnail option

[jonnyowenpowell]

What does this PR do?

This PR adds a new screencapture.show-thumbnail option to the system.defaults configuration.

This option was found by observing how the output of defaults read com.apple.screencapture changed when the 'Show Floating Thumbnail' item was checked and unchecked in the 'Options' menu on the controls bar shown when '⌘ ⇧ 5' (default shortcut) is pressed in MacOS. It determines whether or not a thumbnail will be shown in the bottom right corner of the screen before the screen capture is persisted.

Users may wish to control this option to avoid waiting for the thumbnail to auto-close before accessing the capture.

How was this tested?

I tested this by setting my nix-darwin input to the source branch as part of a flake including nix-darwin and home-manager, running on MacOS Sonoma 14.5 (23F79), and toggling the configuration option on and off between switching generations.

Resources

The UI option (apologies for the poor quality - it's not possible to screen capture the screen capture UI!):

My defaults output when 'Show Floating Thumbnail' enabled :

> defaults read com.apple.screencapture
{
    "last-analytics-stamp" = "739009399.177134";
    "last-selection" =     {
        Height = "837.75";
        Width = 1296;
        X = 216;
        Y = "139.625";
    };
    "last-selection-display" = 0;
    location = "~/Downloads";
    "location-last" = "~/Downloads";
    target = file;
}

My defaults output when 'Show Floating Thumbnail' disabled :

> defaults read com.apple.screencapture
{
    "last-analytics-stamp" = "739009399.177134";
    "last-selection" =     {
        Height = "837.75";
        Width = 1296;
        X = 216;
        Y = "139.625";
    };
    "last-selection-display" = 0;
    location = "~/Downloads";
    "location-last" = "~/Downloads";
    "show-thumbnail" = 0;
    target = file;
}

[Samasaur1]

Does setting this option (either via the UI or via nix-darwin) affect the behavior of the macOS screenshot tool when you use ⇧⌘3 or ⇧⌘4?

[jonnyowenpowell]

Does setting this option (either via the UI or via nix-darwin) affect the behavior of the macOS screenshot tool when you use ⇧⌘3 or ⇧⌘4?

Yes, it does!