policy-node: make streams follow default sink/src changes

The motivation is to have both existing and new streams that have been
created without explicit target node, to be directed to the same devices
at all times. To achieve this:

Consider what find_node returns as the "default node".

Consider streams that auto+reconnect and don't have an explicit target,
as following default.  In rescan, if the default node has changed,
relink such streams to the new default.

Remove the old code that explicitly moved streams when the default.*
metadata changed, as it implements a similar thing but in a less robust
way (may fail to do what's intended, because the default device
metadata is commonly unset, or the metadata callback comes before
session_create has seen the new nodes).

Enable this feature based on "policy-node.streams-follow-default"
property. Allow setting the property also by loading the
streams-follow-default module. Enable it by default only for the
with-pulseaudio module group.

src/examples/media-session/streams-follow-default.c

@@ -0,0 +1,46 @@
+/* PipeWire
+ *
+ * Copyright © 2021 Pauli Virtanen
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ */
+
+/*
+ * Instruct policy-node to move streams when the default sink/sources (either explicitly set
+ * via metadata, or determined from priority) changes, and the stream does not have an
+ * explicitly specified target node.
+ *
+ * This is done by just setting a session property flag, and policy-node does the rest.
+ */
+
+#include "config.h"
+
+#include "pipewire/pipewire.h"
+#include "extensions/metadata.h"
+
+#include "media-session.h"
+
+#define KEY_NAME	"policy-node.streams-follow-default"
+
+int sm_streams_follow_default_start(struct sm_media_session *session)
+{
+	pw_properties_set(session->props, KEY_NAME, "true");
+	return 0;
+}