Move EffectWindowImpl ownership to WindowItem
EffectWindowImpl affects rather the WindowItem, so move its ownership to it. This encapsulates compositing setup so it's effectively same as creating a window item, except some annoyances on X11.
EffectWindowImpl affects rather the WindowItem, so move its ownership to it. This encapsulates compositing setup so it's effectively same as creating a window item, except some annoyances on X11.