Use of the new cache function, setting some addons in the "deprecated" status

* The "privacy_image_cache" functions were used in the past in the addons "buffer" and "fbpost". This is now changed to the new cache function.
* editplain, page and privacy_image_cache are set to the "unsupported" status, since they are deprecated.
* gpluspost generated the RSS feed data even when the commercial plugin was present. (In future the RSS part will be dropped, since the buffer addon can generate much better G+ posts)