implement demolish button

src/ui/GameWorldView.h

@@ -47,6 +47,7 @@ signals:
     void gameOver();
     void builderModeExited();
     void escapeMenuRequested();
+    void demolishModeChanged(bool active);
 
 public:
     double gameSpeed() const;
@@ -54,6 +55,7 @@ public:
 public slots:
     void enterBuilderMode(BuildingType type);
     void exitBuilderMode();
+    void toggleDemolishMode();
     void setGameSpeed(double multiplier);
     void resetForNewGame();