Cleanup: use override specifier everywhere, and enable gcc warning to check for it.

src/script/functions/regex.cpp

@@ -19,8 +19,8 @@ DECLARE_POINTER_TYPE(ScriptRegex);
 /// A regular expression for use in a script
 class ScriptRegex : public ScriptValue, public Regex {
 public:
-  virtual ScriptType type() const { return SCRIPT_REGEX; }
-  virtual String typeName() const { return _("regex"); }
+  ScriptType type() const override { return SCRIPT_REGEX; }
+  String typeName() const override { return _("regex"); }
   
   ScriptRegex(const String& code) {
     assign(code);

src/script/script_manager.hpp

@@ -107,6 +107,6 @@ private:
   
 protected:
   /// Respond to actions by updating scripts
-  void onAction(const Action&, bool undone);
+  void onAction(const Action&, bool undone) override;
 };