CSS and JS adapted from the Overview window close buttons, but with some
style tweaks requested by the designers.
Since the screenshot UI is long-lived (it's created once at startup,
rather than every time it's opened), we need to refresh the close button
position, as it can change at runtime. Subscribing to preference changes
seems to be skipped for bindings generation in Mutter, but simply
refreshing upon opening the UI should do the job.
Closes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4997
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2147>
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