remove 'squares' field from app type, and refactor most functions in common-update.sml to use layer tree instead of squares vector

fcore/normal-mode.sml

@@ -333,11 +333,7 @@ struct
     end
 
   fun useSquares (model, squares, canvasWidth, canvasHeight) =
-    let
-      val model = AppWith.useSquares (model, squares, canvasWidth, canvasHeight)
-    in
-      updateCanvas (model, canvasWidth, canvasHeight)
-    end
+    raise Fail "todo: reimplement"
 
   fun enterBrowseMode model =
     let