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/jack.conf.in

@@ -0,0 +1,52 @@
+# Daemon config file for PipeWire JACK clients version @VERSION@ #
+
+properties = {
+    ## Configure properties in the system.
+    #mem.warn-mlock =		false
+    #mem.allow-mlock =		true
+    #mem.mlock-all =		false
+    #log.level =		2
+}
+
+spa-libs = {
+    ## <factory-name regex> = <library-name>
+    #
+    # Used to find spa factory names. It maps an spa factory name
+    # regular expression to a library name that should contain
+    # that factory.
+    #
+    support.* =		support/libspa-support
+}
+
+modules = {
+    ##  <module-name> = { [args = { <key>=<value> ... }]
+    #                     [flags = [ [ifexists] [nofail] ]}
+    #
+    # Loads a module with the given parameters.
+    # If ifexists is given, the module is ignored when it is not found.
+    # If nofail is given, module initialization failures are ignored.
+    #
+    #
+    # Uses RTKit to boost the data thread priority.
+    libpipewire-module-rtkit  = {
+        args = {
+            #nice.level = -11
+            #rt.prio = 20
+            #rt.time.soft = 200000
+            #rt.time.hard = 200000
+	}
+        flags = [ ifexists nofail ]
+    }
+
+    # The native communication protocol.
+    libpipewire-module-protocol-native = null
+
+    # Allows creating nodes that run in the context of the
+    # client. Is used by all clients that want to provide
+    # data to PipeWire.
+    libpipewire-module-client-node = null
+
+    # Allows applications to create metadata objects. It creates
+    # a factory for Metadata objects.
+    libpipewire-module-metadata = null
+}