Route app save decisions through app core

src/app.h

@@ -24,6 +24,7 @@
 #include "node_input_box.h"
 #include "node_panel_animation.h"
 #include "layout.h"
+#include "app_core/document_session.h"
 
 #if defined(__OBJC__) && defined(__IOS__)
 #import <Foundation/Foundation.h>
@@ -248,6 +249,7 @@ public:
     void dialog_usermanual();
     void dialog_changelog();
     void dialog_about();
+    void save_document(pp::app::DocumentSaveIntent intent);
     void dialog_newdoc();
     void dialog_save();
     void dialog_save_ver();