Prevents compiler warnings such as this:
```
/Users/Will/CLionProjects/OpenMW/apps/openmw/mwgui/windowbase.hpp:65:22: warning: 'onOpen' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
virtual void onOpen();
^
/Users/Will/CLionProjects/OpenMW/apps/openmw/mwgui/windowbase.hpp:38:22: note: overridden virtual function is here
virtual void onOpen() {}
^
```