style.php cannot be properly cached

Right now style.php is fairly intensive, it doesn't attempt to mitigate this by using any validity negotiation based on modification time. However there is still another problem, the SID is appended to each request making the URI unique for every session.

Proposed solutions is to remove the SID and actually use the passed lang= variable. Age based negotiation of freshness could also be added but significant benefits will be made from using persistent URIs.