fromgplus: Picture upload/usage of Object-Type and GUID

**Important:** This pull request contains programming that relies upon this pull request: https://github.com/friendica/friendica/pull/1055

* Pictures that are imported via fromgplus are now uploaded as wall photos. This is much better for privacy protection
* The "object type" (activity streams) is now set when posts are imported. This is work in progress and will be refined in the future
* The GUID is now used in notification URLs.