move item setting to a node flag

app/node/project/sequence/sequence.cpp

@@ -34,6 +34,8 @@ const QString Sequence::kTrackInputFormat = QStringLiteral("track_in_%1");
 
 Sequence::Sequence()
 {
+  SetFlag(kIsItem);
+
   // Create TrackList instances
   track_lists_.resize(Track::kCount);
 

app/node/project/sequence/sequence.h

@@ -87,11 +87,6 @@ public:
 
   static const QString kTrackInputFormat;
 
-  virtual bool IsItem() const override
-  {
-    return true;
-  }
-
 protected:
   virtual void InputConnectedEvent(const QString &input, int element, Node *output) override;