implement entire project in nodes

Project items are now represented directly in nodes opening up more versatility and possibilities. This is the first iteration of this and will be buggy. Need to test thoroughly.

app/dialog/export/export.h

@@ -40,7 +40,7 @@ class ExportDialog : public QDialog
 {
   Q_OBJECT
 public:
-  ExportDialog(ViewerOutput* viewer_node, TimelinePoints* points = nullptr, QWidget* parent = nullptr);
+  ExportDialog(Sequence* sequence, QWidget* parent = nullptr);
 
 protected:
   virtual void closeEvent(QCloseEvent *e) override;
@@ -51,8 +51,7 @@ private:
 
   ExportParams GenerateParams() const;
 
-  ViewerOutput* viewer_node_;
-  TimelinePoints* points_;
+  Sequence* sequence_;
 
   ExportFormat::Format previously_selected_format_;