If the ShellClient got created for a Qt internal window, we try to
find the QWindow and if we get one, we use the geometry directly as
it got set by KWin in the first place.

Also a windowId() is added to ShellClient which can be used by the
effect system to find an EffectWindow. If it's an internal QWindow
we just use that window id. For other clients we still need some
smart solution.