qgs3dmapcanvaswidget: Display a message to report export success state

With this change, the messagebar is used to show a message if the
export operation failed or succeeded.

src/app/3d/qgs3dmapcanvaswidget.cpp

@@ -568,7 +568,18 @@ void Qgs3DMapCanvasWidget::exportScene()
   layout->addWidget( &exportWidget, 1 );
   layout->addWidget( buttons );
   if ( dlg.exec() )
-    exportWidget.exportScene();
+  {
+    const bool success = exportWidget.exportScene();
+    const QString exportFilePath = QDir( exportSettings.sceneFolderPath() ).filePath( exportSettings.sceneName() + QStringLiteral( ".obj" ) );
+    if ( success )
+    {
+      mMessageBar->pushMessage( tr( "Export 3D scene" ), tr( "Successfully exported scene to <a href=\"%1\">%2</a>" ).arg( QUrl::fromLocalFile( exportFilePath ).toString(), QDir::toNativeSeparators( exportFilePath ) ), Qgis::MessageLevel::Success, 0 );
+    }
+    else
+    {
+      mMessageBar->pushMessage( tr( "Export 3D scene" ), tr( "Unable to export scene to <a href=\"%1\">%2</a>" ).arg( QUrl::fromLocalFile( exportFilePath ).toString(), QDir::toNativeSeparators( exportFilePath ) ), Qgis::MessageLevel::Warning, 0 );
+    }
+  }
 }
 
 void Qgs3DMapCanvasWidget::onMainCanvasLayersChanged()