Fix a warning and a build error on macOS

See issue #1979.

src/MainWindow.cpp

@@ -124,7 +124,7 @@ void MainWindow::init()
 #ifdef Q_OS_MACX
     // Add OpenGL Context for macOS
     QOpenGLWidget *ogl = new QOpenGLWidget(this);
-    ui->horizontalLayout->addWidget(ogl);
+    ui->verticalLayout->addWidget(ogl);
     ogl->setHidden(true);
 #endif
 

src/MainWindow.h

@@ -6,7 +6,7 @@
 #include <memory>
 #include <QMainWindow>
 
-class BrowseDataTableSettings;
+struct BrowseDataTableSettings;
 class DbStructureModel;
 class EditDialog;
 class ExtendedTableWidget;