Save screenshots to clipboard

In the case of saving a screenshot to the clipboard, gnome-shell still does provide a temporary file for it, so gsd-media-keys can save it to the clipboard. This seems extra work since gnome-shell has direct access to the clipboard, and is conceptually wrong since it's a daemon doing things usually reserved to UIs (at least on Wayland).

These commits make the screenshot machinery work on streams (moving filename guessing to JS), and allows saving these directly to the clipboard from the compositor side.