Implement config/state file handling

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.

src/daemon/media-session.d/meson.build

@@ -0,0 +1,21 @@
+conf_files = [
+  [ 'bluez-monitor.conf', 'bluez-monitor.conf' ],
+  [ 'v4l2-monitor.conf', 'v4l2-monitor.conf' ],
+  [ 'media-session.conf', 'media-session.conf' ],
+  [ 'alsa-monitor.conf', 'alsa-monitor.conf' ],
+  [ 'with-jack', 'with-jack' ],
+  [ 'with-pulseaudio', 'with-pulseaudio' ],
+]
+
+foreach c : conf_files
+  configure_file(input : c.get(0),
+                 output : c.get(1),
+                 configuration : conf_config,
+                 install_dir : join_paths(conf_install_dir, 'media-session.d'))
+endforeach
+
+install_data(
+  sources : [
+    'with-jack',
+    'with-pulseaudio' ],
+  install_dir : join_paths(conf_install_dir, 'media-session.d'))