DisplaySnackbar

Shows a snackbar with message, by broadcasting a display command on the screen channel. duration maps onto Material's short/long/indefinite durations, and actionLabel adds an action button when non-null. Does not consume incomingData. Dispatches onSuccess (no data) right after the command is broadcast, before the snackbar has resolved — the broadcast is fire-and-forget, so this fires regardless of what the snackbar does next; later, onSnackbarAction (no data) when the user presses the action button, or onSnackbarDismissed (no data) when the snackbar goes away without its action being pressed.

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