restore callbacks to WebTwainEnv

Author: kyle-ds

info/api/Dynamsoft_WebTwainEnv.md

@@ -62,8 +62,11 @@ interface Dynamsoft.DWT {
     Load(): void;
     Unload(): void;
     RemoveAllAuthorizations: function () {};    
+    OnWebTwainNotFound: function () {};
     OnWebTwainPostExecute: function () {};
     OnWebTwainPreExecute: function () {};
+    OnWebTwainReady: function () {};
+    OnWebTwainWillInit: function () {};
 }
 
 /**