Work around problem of restored QQuickWindows not being visible

A declarative KWin script needs to register the QQuickWindows it is
using in the KWin object. This method ensures that the QQuickWindow
will destroy the platfrom window once it gets hidden. So the next
time the QQuickWindow is shown a new platform window is created.

As can be seen in the OSD this is not really nice, therefore a
KWin.Dialog should be created which takes care of these steps.