better attachment between labels and tracks

timeline/tracklist.cpp

@@ -37,6 +37,8 @@ void TrackList::AddTrack()
 {
   Track* track = new Track(this, type_);
   tracks_.append(track);
+
+  emit TrackCountChanged();
 }
 
 void TrackList::RemoveTrack(int i)
@@ -45,6 +47,8 @@ void TrackList::RemoveTrack(int i)
     return;
   }
   tracks_.removeAt(i);
+
+  emit TrackCountChanged();
 }
 
 Track *TrackList::First()
@@ -52,6 +56,11 @@ Track *TrackList::First()
   return tracks_.first();
 }
 
+Track *TrackList::Last()
+{
+  return tracks_.last();
+}
+
 int TrackList::TrackCount()
 {
   return tracks_.size();