Console: Make isOpen() a const member function

This member function doesn't modify internal member state, so it can be
turned into a const qualified member function.

hecl/include/hecl/Console.hpp

@@ -108,6 +108,6 @@ public:
   void dumpLog();
   static Console* instance();
   static void RegisterLogger(Console* con);
-  bool isOpen() { return m_state == State::Opened; }
+  bool isOpen() const { return m_state == State::Opened; }
 };
 } // namespace hecl