Extract document open unsaved prompt helper

src/legacy_document_open_services.cpp

@@ -102,6 +102,18 @@ void prompt_import_brush_package(
     };
 }
 
+void prompt_discard_unsaved_project(App& app, const pp::app::DocumentOpenRoute& route)
+{
+    auto mb = app.message_box(
+        "Unsaved document",
+        "Do you want to close the unsaved document before opening the file?",
+        true);
+    mb->on_submit = [&app, route](Node* target) {
+        open_legacy_project(app, route);
+        pp::panopainter::close_legacy_dialog_node(*target);
+    };
+}
+
 class LegacyDocumentOpenServices final : public pp::app::DocumentOpenServices {
 public:
     explicit LegacyDocumentOpenServices(App& app) noexcept
@@ -136,15 +148,7 @@ public:
 
     void prompt_discard_unsaved_project(const pp::app::DocumentOpenRoute& route) override
     {
-        auto* app = &app_;
+        pp::panopainter::prompt_discard_unsaved_project(app_, route);
-        auto mb = app_.message_box(
-            "Unsaved document",
-            "Do you want to close the unsaved document before opening the file?",
-            true);
-        mb->on_submit = [app, route](Node* target) {
-            open_legacy_project(*app, route);
-            pp::panopainter::close_legacy_dialog_node(*target);
-        };
     }
 
 private: