OptionalmoveOptionalonCalled for a command the runner does not know (a converter's transfer, picture,
sound, ...), and for a goto in a straight run(), which ends that list. Without
it an unknown command is skipped silently; a game that converts foreign event data
passes one to make every dropped command loud.
shows a line or a question; two-d/ui.messageBoxPresenter is the standard one
carries out a move command; the runner itself does not know what "moving" means