SystemDialog: make it a real window
We want it to be a standalone window, but the root item was not Window; we were relying on implicit behavior to make it be a window. This caused various issues including the window not getting a Close button on X11.
Let's be explicit and make its root item be a real Window instance. We did this to fix a similar issue in the KNewStuff dialog; see frameworks/knewstuff@ea19fa6e.
Please review and test carefully as I am not 100% sure this is the right implementation. Everything seems to work in my testing, but I'm not super familiar with this code.