Asynchronous version of downloadRequested

WebEngineProfile::downloadRequested expects a decision of the download to be made during the execution of downloadRequested handler. This is a little difficult to use in QML API because there is no way to put custom event loop somewhere without creating some c++ classes to help with that.

Perhaps a version of downloadRequested could be added which would run QEventLoop internally until a decision has been made unless it is possible change the logic of downloadRequested (perhaps by adding a function to not proceed immediately after returning from the function and instead run QEventLoop there waiting for the decision).