[WinRT/UWP] Implement QEventDispatcherWinRT::registerEventNotifier / registerSocketNotifier

With UWP reindroducing WinSock, not having QSocketNotifier is bit of a drag for supporting older socket code that would otherwise now work fine with UWP. Not having is also the only stopper from enabling the faster/stabler existing win32 QNativeSocketEngine and unifying the network handling.

Not having QWinEventNotifier is very inconventent. It makes fixing broken thread handling in the WASAPI plugin (see QTBUG-55894) very difficult for example.