core: Add PicasaWeb support to GDataGoaAuthorizer

GOA has supported photos for a long time. Grab PicasaWeb
authentication if GOA has already got a token supporting it.

This bumps the GOA dependency to 3.8.

https://bugzilla.gnome.org/show_bug.cgi?id=731269
core: Refresh authoriser on receiving SOUP_STATUS_FORBIDDEN

Previously the authoriser was only refreshed on receiving a
SOUP_STATUS_UNAUTHORIZED response from the server. This worked fine for
Google Documents, but it seems that PicasaWeb returns FORBIDDEN instead.
The authoriser was not being refreshed, and hence the user was seeing
access denied messages even though they were technically logged in.

https://bugzilla.gnome.org/show_bug.cgi?id=731269
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.