Modified play forward and play backward to be require the output of each
other in their usages in HistoryChangeForwarder, to ensure that the
context-specific revertibles are used - NOTICE, ONLY THE SKELETON PART
IS IMPLEMENTEED; I.E. THE PLAY FORWARD AND PLAY BACK METHODS AND
COMMANDS DO NOT CURRENTLY CREATE AND OUTPUT THE ABOVEMENTIONED KIND OF
REVERTIBLES.

Signed-off-by: Jakob Brandsborg Ehmsen <jakobehmsen.github@gmail.com>