Use std::optional instead of sentinel values for absent data

src/ui/ShipLayoutDialog.h

@@ -47,7 +47,7 @@ public:
     struct CellInfo
     {
         bool buildable;
-        int moduleIndex; // -1 if empty
+        std::optional<int> moduleIndex;  // nullopt if empty
     };
 
 private:
@@ -69,7 +69,12 @@ private:
     std::vector<PlacedModule> m_placedModules;
     std::vector<std::vector<CellInfo>> m_grid;
 
-    int m_activeModuleIndex; // -1 = remove mode, -2 = no selection
+    // The module to place, as an index into config modules; nullopt when no
+    // module is selected for placement. m_removeMode is a separate mode in which
+    // clicking a cell removes the module there (mutually exclusive with a
+    // selected module).
+    std::optional<int> m_activeModuleIndex;
+    bool m_removeMode = false;
     Rotation m_currentRotation;
 
     std::vector<QPushButton*> m_moduleButtons;