add functionality to flip drawing horizontally

fcore/normal-mode.sml

@@ -369,6 +369,15 @@ struct
     in (model, [])
     end
 
+  fun flipHorizontally (model: app_type) =
+    let
+      val {layerTree, arrowX, arrowY, ...} = model
+      val layerTree = LayerTree.flipHorizontally layerTree
+      val model = AppWith.layerTree (model, layerTree, arrowX, arrowY)
+    in
+      getDrawMessage (model, [])
+    end
+
   fun update (model: app_type, inputMsg) =
     case inputMsg of
       MOUSE_MOVE {x = mouseX, y = mouseY} =>
@@ -388,6 +397,7 @@ struct
     | KEY_W => updateCanvasWidth model
     | KEY_H => updateCanvasHeight model
     | KEY_M => enterMoveMode model
+    | KEY_F => flipHorizontally model
     | RESIZE_WINDOW {width, height} => resizeWindow (model, width, height)
     | UNDO_ACTION => undoAction model
     | REDO_ACTION => redoAction model