winrt: emit application activation

Previously the user only got informed about the application getting
inactive or hidden, but not when the application was reactivated again.
This can cause problems with applications using for instance a camera as
that gets disabled when not being active and never return to a active
state again.

Change-Id: I5c1d8750db8e75043ecf261616a0bc98434a3863
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Andrew Knight <andrew.knight@intopalo.com>
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>

src/plugins/platforms/winrt/qwinrtscreen.cpp

@@ -1132,6 +1132,8 @@ HRESULT QWinRTScreen::onActivated(ICoreWindow *, IWindowActivatedEventArgs *args
         return S_OK;
     }
 
+    QWindowSystemInterface::handleApplicationStateChanged(Qt::ApplicationActive);
+
     // Activate topWindow
     if (!d->visibleWindows.isEmpty()) {
         Qt::FocusReason focusReason = activationState == CoreWindowActivationState_PointerActivated