Make methods to load_config and load/save state. For now the config
and state directories are the same but it might not be. Implement
the search path for all config/state files as:

  $XDG_CONFIG_HOME/[$prefix]/$name
  $HOME/.config/[$prefix]/$name
  $PIPEWIRE_CONFIG_DIR/pipewire/[$prefix]/$name
  /etc/pipewire/[$prefix]/$name

Make some config files for jack and RT clients. Make pw-cat use the
client-rt config.

Use core state and config management in media-session.
Move all session manager state and config files to the build dir and
set the PIPEWIRE_CONFIG_DIR to this build dir.