Implement QWindow::setWindowIcon

Implement QWindow::setWindowIcon

Since QIcon has been moved back to QtGui, QWindow::setWindowIcon can
use it. That way, the api is exactly the same as in QWidgets and one
can deal properly with multi-sized icon.
I added a getter so the api is consistent with QWidget
(Maybe there should be properties for windowIcon and windowTitle)

Change-Id: I2f463dbe39673f41a3201ef8fed27b3fcac2125f
Reviewed-by: Girish Ramakrishnan <girish.1.ramakrishnan@nokia.com>