[gui:] added csv-export placeholder button

MainWindow.axaml.cs

@@ -423,4 +423,9 @@ public partial class MainWindow : Window
             Settings.Instance.NumCoursesPerSemester = Convert.ToInt32(NudSportMaxPerSemester.Value);
         } catch {}
     }
+
+    private void BtnExportCourseCSV_OnClick(object? sender, RoutedEventArgs e)
+    {
+        // export as csv here
+    }
 }