phoc_layer_surface_effects: Add a way for client to acknowledge drag state
Currently there's no way for the client to signalize that it acknowledged updated drag state and updated its other state to match it. This leads to a glitch seen in !380 (merged), when the compositor already finished unfolding animation, but phosh (with phosh!1085 (merged)) didn't update its keyboard interactivity yet, leading to a quick and rather annoying blink: