timeline: handle users deleting tentative subtitle track

Fixes potential crash if users try to do this

app/widget/timelinewidget/trackview/trackview.cpp

@@ -121,9 +121,13 @@ void TrackView::TrackHeightChanged(int index, int height)
 
 void TrackView::InsertTrack(Track *track)
 {
+  TrackViewItem *tvi = new TrackViewItem(track);
+
+  connect(tvi, &TrackViewItem::AboutToDeleteTrack, this, &TrackView::AboutToDeleteTrack);
+
   splitter_->Insert(track->Index(),
                     track->GetTrackHeightInPixels(),
-                    new TrackViewItem(track));
+                    tvi);
 }
 
 void TrackView::RemoveTrack(Track *track)

app/widget/timelinewidget/trackview/trackview.h

@@ -40,6 +40,9 @@ public:
   void ConnectTrackList(TrackList* list);
   void DisconnectTrackList();
 
+signals:
+  void AboutToDeleteTrack(Track *track);
+
 protected:
   virtual void resizeEvent(QResizeEvent *e) override;
 

app/widget/timelinewidget/trackview/trackviewitem.cpp

@@ -145,6 +145,7 @@ void TrackViewItem::ShowContextMenu(const QPoint &p)
 
 void TrackViewItem::DeleteTrack()
 {
+  emit AboutToDeleteTrack(track_);
   Core::instance()->undo_stack()->push(new TimelineRemoveTrackCommand(track_));
 }
 

app/widget/timelinewidget/trackview/trackviewitem.h

@@ -38,6 +38,9 @@ public:
   TrackViewItem(Track* track,
                 QWidget* parent = nullptr);
 
+signals:
+  void AboutToDeleteTrack(Track *track);
+
 private:
   QPushButton* CreateMSLButton(const QColor &checked_color) const;