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-init.sml

@@ -45,7 +45,7 @@ struct
       , g = 0
       , b = 0
       , a = 1
-      , layer = 0
+      , layer = LayerTree.minKey
       , layerTree = layerTree
       , modalNum = 0
       }