Implement config-driven unlock groups so that multiple things can be unlocked at once (including buildings)

src/ui/BuildButtonGrid.h

@@ -13,6 +13,7 @@
 #include "DemolishModeChangedEvent.h"
 #include "EventHandler.h"
 #include "GameConfig.h"
+#include "UnlockedBuildingsChangedEvent.h"
 
 class QPushButton;
 class Simulation;
@@ -21,7 +22,8 @@ class BuildButtonGrid : public QWidget,
                         public CombinedEventHandler<BuilderModeExitedEvent,
                                                     DemolishModeChangedEvent,
                                                     BuildHotkeyPressedEvent,
-                                                    BuildingBlocksChangedEvent>
+                                                    BuildingBlocksChangedEvent,
+                                                    UnlockedBuildingsChangedEvent>
 {
     Q_OBJECT
 
@@ -37,10 +39,15 @@ private:
     // be afforded, it exits builder mode so the button does not stay selected.
     void updateAffordability();
 
+    // Re-evaluates which build buttons are visible from the current per-building
+    // unlock state (REQ-LOCK-BUILDING); a locked building type's button is hidden.
+    void updateVisibility();
+
     void handleEvent(std::shared_ptr<const BuilderModeExitedEvent> event) override;
     void handleEvent(std::shared_ptr<const DemolishModeChangedEvent> event) override;
     void handleEvent(std::shared_ptr<const BuildHotkeyPressedEvent> event) override;
     void handleEvent(std::shared_ptr<const BuildingBlocksChangedEvent> event) override;
+    void handleEvent(std::shared_ptr<const UnlockedBuildingsChangedEvent> event) override;
 
 private slots:
     void onBuildButton(int index);