scene/item: add ItemEffect

Right now it's just a helper to mark items as being affected by some effect, to prevent direct scanout of the relevant item without needing to block direct scanout for the whole screen - basically to fix !6202 (closed) sometimes disabling blur for some items. It also removes the need to special case some effects to not block direct scanout like in !6086 (merged).

As a future plan for this, I think it should be extended so that effects can only change the rendering for a given window if they create an ItemEffect. Effects that effectively add windows - like the overview or the fps effect - should insert an overlay item into the scene (and !6193 (merged) probably moved to be built into the renderer in some form, or special cased) and Effect::blocksDirectScanout removed or at least set to false by default.