MIDI tracer: disable trace when window is closed

This fixes a bug with some MIDI port constantly tracing,
because trace_parser is never unset.

gtk2_ardour/midi_tracer.cc

@@ -128,6 +128,21 @@ MidiTracer::MidiTracer ()
 
 MidiTracer::~MidiTracer()
 {
+	disconnect ();
+}
+
+void
+MidiTracer::on_show ()
+{
+	ArdourWindow::on_show ();
+	collect_toggle ();
+}
+
+void
+MidiTracer::on_hide ()
+{
+	ArdourWindow::on_hide ();
+	disconnect ();
 }
 
 void

gtk2_ardour/midi_tracer.h

@@ -53,6 +53,10 @@ public:
 	MidiTracer ();
 	~MidiTracer();
 
+protected:
+	void on_show ();
+	void on_hide ();
+
 private:
 	Gtk::TextView text;
 	Gtk::ScrolledWindow scroller;