Consider supporting callbacks in modifiers

[JakeWharton]

Already working in regular Redwood, but protocol Redwood does not support it as it tries to serialize them which obviously fails. Within a widget we could have a monotonic integer with which we associate each lambda and then use that for sending the events backwards. Just gotta be careful about correctly freeing old lambdas if the modifier stack changes.

[swankjesse]

Would have a nice benefit to at least one use in Cash App. (Emoji Picking.)