Promise might leak memory

When the callback object from a Promise object is kept in C++ side like the slot for timeout in the example, and the callback object is invoked using QJSValue::call, the arguments might stay in memory and cause a memory leak.

STR:

1. Run the example and click the window area. See if any "destroyed" shows up in the console.
2. Without setting Qt::SingleShotConnection in the connection, the MemoryEater object passed to the callback object is not GC'd.