windowPreview: Consider chrome overlaps when offscreening for opacity

The icon and close button might be overlapping the window actor but were not considered in has_overlaps() which gets used to decide whether to offscreen the actor for transparency. Since currently the icon is visible when the preview is dragged and the whole actor is turned transparent, the opacity will not be applied to everything as a whole but the child actors individually. This leads to the window becoming visible behind the icon.

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