Ensures signal emission for resolved/rejected promises

Ensures that the resolved()/rejected() signals are emitted even if the Promise is created with a resolved/rejected Deferred. Accordingly, it is ensured that a NetworkDeferred is resolved/rejected when created with a finished QNetworkReply.

The downside is that an event loop is needed to emit the signals. However, I think this is a reasonable drawback when working with an asynchronous data library.