winrt: Fix file dialog handling for Windows Phone

Windows Phone needs to use activation targets to properly handle the
return value of a file dialog. 0f9ca217d0
introduced launching an app via different modes, but broke above use-
case for Windows Phone.

Hence, we first check if a dispatcher exists and use this one to forward
the activation.

Task-number: QTBUG-54342
Change-Id: If9dd2df9a45e9aa104775530c695325fe6f684f2
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>

src/winmain/qtmain_winrt.cpp

@@ -198,6 +198,14 @@ public:
 
 private:
     HRESULT activatedLaunch(IInspectable *activateArgs) {
+        // Check if an application instance is already running
+        // This is mostly needed for Windows Phone and file pickers
+        QAbstractEventDispatcher *dispatcher = QCoreApplication::eventDispatcher();
+        if (dispatcher) {
+            QCoreApplication::postEvent(dispatcher, new QActivationEvent(activateArgs));
+            return S_OK;
+        }
+
         QCoreApplication *app = QCoreApplication::instance();
 
         // Check whether the app already runs