add redo stack, and adjust functions to clear it when new triangle or triangle_stage is added; when an undo function is called, the undoHd becomes the redoHd, and undo becomes undoTl, which is expected behaviour. Still need to implement REDO_ACTION though.

functional-core/app-init.sml

@@ -24,6 +24,7 @@ struct
       , yClickPoints = yClickPoints
       , graphLines = graphLines
       , undo = []
+      , redo = []
       , mouseX = 0.0
       , mouseY = 0.0
       }