Make i3bar get its config from i3 via IPC

Make i3bar get its config from i3 via IPC

In order to not duplicate configuration options and make stuff confusing, we
dropped the commandline flags (except for socket_path and bar_id). This means
that you *have to* specify bar_id when starting i3bar. The best way is to let
i3 start i3bar, which it will do automatically for every bar {} configuration
block it finds.