cache: heavily reworked exporting for new system

Also cleans up cache in general and moves it to the Task infrastructure.

app/dialog/export/export.h

@@ -42,9 +42,6 @@ class ExportDialog : public QDialog
 public:
   ExportDialog(ViewerOutput* viewer_node, QWidget* parent = nullptr);
 
-public slots:
-  virtual void accept() override;
-
 protected:
   virtual void closeEvent(QCloseEvent *e) override;
 
@@ -123,6 +120,8 @@ private slots:
 
   void UpdateViewerDimensions();
 
+  void StartExport();
+
 };
 
 OLIVE_NAMESPACE_EXIT