Add track playlist-state function for undo/redo

libs/ardour/ardour/track.h

@@ -66,6 +66,8 @@ public:
 
 	bool set_processor_state (XMLNode const& node, int version, XMLProperty const* prop, ProcessorList& new_order, bool& must_configure);
 
+	XMLNode& playlist_state () const;
+
 	bool declick_in_progress () const;
 
 	bool can_record();

libs/ardour/track.cc

@@ -162,6 +162,23 @@ Track::chan_count_changed ()
 	ChanCountChanged (); /* EMIT SIGNAL */
 }
 
+XMLNode&
+Track::playlist_state () const
+{
+	XMLNode* node = new XMLNode("Route");
+	node->set_property("version", CURRENT_SESSION_FILE_VERSION);
+
+	if (_playlists[DataType::AUDIO]) {
+		node->set_property (X_("audio-playlist"), _playlists[DataType::AUDIO]->id().to_s());
+	}
+
+	if (_playlists[DataType::MIDI]) {
+		node->set_property (X_("midi-playlist"), _playlists[DataType::MIDI]->id().to_s());
+	}
+
+	return *node;
+}
+
 XMLNode&
 Track::state (bool save_template) const
 {