add icons for build buttons

src/ui/BuildButtonGrid.h

@@ -2,6 +2,7 @@
 
 #include <map>
 #include <optional>
+#include <string>
 #include <vector>
 
 #include <QWidget>
@@ -28,7 +29,10 @@ class BuildButtonGrid : public QWidget,
     Q_OBJECT
 
 public:
-    BuildButtonGrid(Simulation* sim, const GameConfig* config, QWidget* parent = nullptr);
+    // iconDir is the directory holding the per-building "<id>.svg" chip icons
+    // (REQ-UI-BUILD-GRID); read from disk at runtime, like the config files.
+    BuildButtonGrid(Simulation* sim, const GameConfig* config,
+                    const std::string& iconDir, QWidget* parent = nullptr);
     ~BuildButtonGrid() override;
 
     void clearActiveButton();
@@ -55,6 +59,7 @@ private slots:
 private:
     Simulation*                      m_sim;
     const GameConfig*                m_config;
+    std::string                      m_iconDir;
     std::vector<BuildingType>        m_types;
     std::vector<QPushButton*>        m_buttons;
     std::map<BuildingType, int>      m_costs;