[wayland] ShellClient can reference an internal QWindow
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.
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