Route document workflow prompts through app core

src/app.h

@@ -250,6 +250,7 @@ public:
     void dialog_changelog();
     void dialog_about();
     void save_document(pp::app::DocumentSaveIntent intent);
+    void continue_document_workflow_after_optional_save(std::function<void()> action);
     void dialog_newdoc();
     void dialog_save();
     void dialog_save_ver();