Introduce private link sharing #5023

Introduce private link sharing #5023

* SocketAPI has COPL_LOCAL_LINK / EMAIL_LOCAL_LINK commands
* The nautilus and dolphing shell integrations show a submenu from which
  one can share as well as access the private link.
* The SocketAPI provides a new GET_STRINGS command to access localized
  strings.
* The private link can also be accessed from the user/group sharing
  dialog.
* The numeric file id is extracted from the full id to create the
  private link url.