community/gnome-user-share: include systemd user units, add to gnome-apps-core

It's gnome-user-share's preferred way to start the daemon. And because it is compatible with superd (available in aports) it has a supported way of starting it, in Alpine with OpenRC.

According to https://apps.gnome.org/_api/components.json it is part of the Core category. Since it is only started with the user unit, it isn't auto-started by default.