Make return value of simple types getters constant

Fixed: #14

src/generator/templates.cpp

@@ -141,7 +141,7 @@ const char *Templates::GetterMessageDefinitionTemplate = "const $type$ &$classna
                                         "    return *m_$property_name$;\n"
                                         "}\n\n";
 
-const char *Templates::GetterTemplate = "$type$ $property_name$() const {\n"
+const char *Templates::GetterTemplate = "const $type$ $property_name$() const {\n"
                                         "    return m_$property_name$;\n"
                                         "}\n\n";