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/task/export/export.cpp

@@ -25,7 +25,7 @@
 
 namespace olive {
 
-ExportTask::ExportTask(ViewerOutput* viewer_node,
+ExportTask::ExportTask(Sequence *viewer_node,
                        ColorManager* color_manager,
                        const ExportParams& params) :
   RenderTask(viewer_node, params.video_params(), params.audio_params()),

app/task/export/export.h

@@ -33,7 +33,7 @@ class ExportTask : public RenderTask
 {
   Q_OBJECT
 public:
-  ExportTask(ViewerOutput *viewer_node, ColorManager *color_manager, const ExportParams &params);
+  ExportTask(Sequence *viewer_node, ColorManager *color_manager, const ExportParams &params);
 
 protected:
   virtual bool Run() override;