implement demolish button

src/ui/BuildButtonGrid.h

@@ -23,6 +23,10 @@ public:
 signals:
     void buildingTypeSelected(BuildingType type);
     void builderModeExited();
+    void demolishModeToggleRequested();
+
+public slots:
+    void setDemolishModeActive(bool active);
 
 private slots:
     void onBuildButton(int index);
@@ -33,4 +37,5 @@ private:
     std::vector<QPushButton*>        m_buttons;
     std::map<BuildingType, int>      m_costs;
     int                              m_activeIndex;
+    QPushButton*                     m_demolishButton;
 };