done with functional-core logic for redo (now just need to send REDO_ACTION on key callback when shortcut is pressed)

message-types/input-msg.sml

@@ -6,6 +6,7 @@ sig
   | MOUSE_LEFT_RELEASE
   | RESIZE_WINDOW of {width: int, height: int}
   | UNDO_ACTION
+  | REDO_ACTION
 end
 
 structure InputMessage :> INPUT_MESSAGE =
@@ -16,4 +17,5 @@ struct
   | MOUSE_LEFT_RELEASE
   | RESIZE_WINDOW of {width: int, height: int}
   | UNDO_ACTION
+  | REDO_ACTION
 end