NeoUI - Full server queuing + automatic joining

On entering a server, a ping-reply is now sent to the server to
get the most up to date players count upon entering. Auto-join
is determined if the server's full when trying to enter, and if
so enters an auto-join state where it'll refresh for 15s checking
count then entering when there's a free slot available. The
auto-join popup bar stays on any states.

* fixes #1298

Author: nullsystem