Lua: Add connection guard

Improves safety by limiting connection lifetime to
lifetime of lua vm.

Change-Id: I508b0814fdda169f5e30d7c3282132e3c622a0a5
Reviewed-by: hjk <hjk@qt.io>

src/plugins/lua/luaengine.h

@@ -37,6 +37,7 @@ struct ScriptPluginSpec
 {
     QString name;
     Utils::FilePath appDataPath;
+    std::unique_ptr<QObject> connectionGuard;
 };
 
 class LUA_EXPORT LuaEngine final : public QObject