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

src/lib/eventsystem/event/UnlockedBuildingsChangedEvent.h

@@ -0,0 +1,13 @@
+#ifndef UNLOCKED_BUILDINGS_CHANGED_EVENT_H
+#define UNLOCKED_BUILDINGS_CHANGED_EVENT_H
+
+#include "Event.h"
+
+// Emitted when the set of unlocked building types changes (REQ-LOCK-BUILDING),
+// i.e. after an unlock group granting a building is awarded (REQ-DEF-SCHEMATIC-DROP)
+// or on Restart. The build button grid re-evaluates which buttons are shown.
+class UnlockedBuildingsChangedEvent : public Event
+{
+};
+
+#endif   // UNLOCKED_BUILDINGS_CHANGED_EVENT_H