qmlscene can start with either an Item or Window as the qml root

qmlscene can start with either an Item or Window as the qml root

Before, it assumed that the root is an Item and needs to have a
Window created.  But it's useful for an application to have a Window
as the root, and it was already possible by writing a different C++
main function (see qtdeclarative/examples/window/window/window.cpp).
It doesn't take much to give qmlscene this flexibility too.

Change-Id: Ie808e78a42074e13aa9d3c87723ec9ac8fdbaf4a
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>