[applets/devicenotifier] Show "remove all" when only one disk mounted
Currently the "Remove all" item appears only when multiple external disks mounted. IMHO it should be available even if there's only one disk mounted.
- I'm getting used to use "Remove all" to unmount all external disks, but often frustrated to not finding it when there's only one disk.
- Generally, "Close all" actions are available when there's a single thing to close. E.g. "Close all files" in text editors (Kate). "Close all tabs" in web browsers.