change BNF (but not implementation of BNF) for parsing, to handle layers, and also change minimum key in layer tree to be 1

fcore/app-with.sml

@@ -746,7 +746,7 @@ struct
         , modalNum
         } = app
 
-      val layer = modalNum
+      val layer = Int.max (modalNum, 1)
     in
       { mode = mode
       , mouseX = mouseX
@@ -770,7 +770,7 @@ struct
       , a = a
       , layer = layer
       , layerTree = layerTree
-      , modalNum = modalNum
+      , modalNum = 0
       }
     end