dim the game behind dialogs and escape menu

src/ui/MainWindow.h

@@ -12,6 +12,7 @@
 #include "EventHandler.h"
 #include "GameOverEvent.h"
 #include "LayoutDialogRequestedEvent.h"
+#include "ModalDimOverlay.h"
 #include "WinEvent.h"
 #include "RecipeSelectionRequestedEvent.h"
 #include "SchematicChoicesAvailableEvent.h"
@@ -76,6 +77,7 @@ private:
     BuildButtonGrid*       m_buildButtonGrid;
     BlueprintPanel*        m_blueprintPanel;
     QWidget*               m_sidePanel;
+    ModalDimOverlay*       m_dimOverlay = nullptr;
 
     std::vector<ShipLayoutBlueprint> m_layoutBlueprints;
     std::shared_ptr<ParsedReplay>    m_replay;  // non-null => view-only playback