Set Qt::AA_DontCreateNativeWidgetSiblings on Windows/Mac.

Prevent creation of native windows for subwidgets, caused
by the welcome screen.

Task-number: QTCREATORBUG-10251

Change-Id: I2b475e963c5dfd1485f0876817c88d80fc2abd25
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>

src/app/main.cpp

@@ -312,6 +312,11 @@ int main(int argc, char **argv)
 
 #if QT_VERSION >= 0x050100
     app.setAttribute(Qt::AA_UseHighDpiPixmaps);
+#  if defined(Q_OS_WIN) || defined(Q_OS_MAC)
+    // Prevent native windows from being created for the sibling widgets of the welcome screen.
+    // Causes flicker on Linux, though.
+    app.setAttribute(Qt::AA_DontCreateNativeWidgetSiblings);
+#  endif
 #endif
 
     // Manually determine -settingspath command line option