qmlscene: don't call show() on the window if it's already showing

That way Component.onCompleted() can optionally do showMaximized()
without being overridden.

Task-number: QTBUG-31260
Change-Id: I6f5c3025efd3b38809be85da4172241695168a6c
Reviewed-by: Nikita Krupenko <krnekit@gmail.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>

tools/qmlscene/main.cpp

@@ -516,7 +516,7 @@ int main(int argc, char ** argv)
                     window->showFullScreen();
                 else if (options.maximized)
                     window->showMaximized();
-                else
+                else if (!window->isVisible())
                     window->show();
             }