implement cost formula for asteroid expansion

src/lib/eventsystem/event/ExpansionCostChangedEvent.h

@@ -0,0 +1,17 @@
+#ifndef EXPANSION_COST_CHANGED_EVENT_H
+#define EXPANSION_COST_CHANGED_EVENT_H
+
+#include "Event.h"
+
+// Fired when the current asteroid-expansion cost changes (REQ-EXP-COST): once at
+// startup and again after each expansion is purchased. Carries the cost in
+// building blocks so the header Expand button can update its caption/enabled
+// state (REQ-UI-EXPAND-BUTTON).
+class ExpansionCostChangedEvent : public Event
+{
+public:
+	explicit ExpansionCostChangedEvent(int cost) : cost(cost) {}
+	const int cost;
+};
+
+#endif   // EXPANSION_COST_CHANGED_EVENT_H