add code to let user change alpha valua

fcore/normal-mode.sml

@@ -259,6 +259,7 @@ struct
   fun updateRed model = (AppWith.r model, [])
   fun updateGreen model = (AppWith.g model, [])
   fun updateBlue model = (AppWith.b model, [])
+  fun updateAlpha model = (AppWith.a model, [])
 
   fun enterBrowseMode model =
     let
@@ -287,6 +288,7 @@ struct
     | KEY_R => updateRed model
     | KEY_G => updateGreen model
     | KEY_B => updateBlue model
+    | KEY_A => updateAlpha model
     | RESIZE_WINDOW {width, height} => resizeWindow (model, width, height)
     | UNDO_ACTION => undoAction model
     | REDO_ACTION => redoAction model