make HeaderBar read the tick, artifacts and boss wave from the simulation instead of event

src/ui/HeaderBar.h

@@ -75,8 +75,8 @@ private:
 
     ItemIconCache*           m_itemIcons;  // Not owned; lives in MainWindow.
 
-    // The simulation is the single source of truth for the block stock and the
-    // expansion cost; the change events are only refresh signals.
+    // The simulation is the single source of truth for everything the header
+    // displays; the change events are only refresh signals.
     const Simulation*        m_sim;
 
     static const double kSpeeds[];