picasaweb: Support <gphoto:access>protected</gphoto:access> in albums

The documentation
    https://developers.google.com/picasa-web/docs/2.0/reference?csw=1#Visibility
doesn’t have any mention of this, so let’s alias it to
GDATA_PICASAWEB_PRIVATE for the moment.

It seems to be set on albums which are marked as visible only to the
owner.