midiview: correctly manage NoteBase lifetimes

MidiView::clear_events() deleted the canvas items corresponding to Notes/Hits,
but did not delete the owner NoteBase objects.

gtk2_ardour/hit.cc

@@ -40,7 +40,9 @@ Hit::Hit (MidiView& region, Item* parent, double size, const std::shared_ptr<Not
 
 Hit::~Hit ()
 {
-	delete _polygon;
+	/* do not delete the visual note here, because that will be handled by
+	 * the parent
+	 */
 }
 
 void

gtk2_ardour/midi_view.cc

@@ -873,8 +873,12 @@ MidiView::clear_events ()
 	// clear selection without signaling or trying to change state of event objects
 	_selection.clear ();
 	clear_ghost_events ();
-	/* This will delete all the NoteBase* in the _events map */
+	/* This will delete all the _visual_note members of the NoteBase* elements in the _events map */
 	_note_group->clear (true);
+	/* The above did not actually delete the the NoteBase* elements, so do that now  */
+	for (auto & [model_note,gui_note] : _events) {
+		delete gui_note;
+	}
 	_events.clear();
 	_patch_changes.clear();
 	_sys_exes.clear();

gtk2_ardour/note.cc

@@ -39,7 +39,9 @@ Note::Note (MidiView& region, ArdourCanvas::Item* parent, const std::shared_ptr<
 
 Note::~Note ()
 {
-	delete _visual_note;
+	/* do not delete the canvas item here, because that will be handled by
+	 * the parent
+	 */
 }
 
 void

gtk2_ardour/note_base.cc

@@ -88,8 +88,7 @@ NoteBase::NoteBase(MidiView& v, bool with_events, const std::shared_ptr<NoteType
 NoteBase::~NoteBase()
 {
 	_view.note_deleted (this);
-
-	delete _text;
+	/* do not delete _text, parent will do so */
 }
 
 void