Ensure that the WindowThumbnail is not trying to render a thumbnail of itself

Recursive window thumbnails would look awesome on the screen, but reality
is that X/OpenGL or $DEITY doesn't like it at all and decided to just
freeze the view.

So let's delay all the redirecting till the WindowThumbnail has been
added to its QQuickWindow and if the window id is the one of the own
window we just render the icon instead.