media-keys: Let the screenshot service save to clipboard itself
This is both broken currently on wayland (since this daemon doesn't request the x11 backend, and wayland clients need a proper surface/serial to request clipboard), and conceptually wrong (we wouldn't like to enable the X11 backend just to let it save to clipboard).
Let gnome-shell handle saving the screenshot to clipboard instead, this also saves us from saving to /tmp just to copy it to the clipboard afterwards. We achieve this by passing an empty string as the filename.