Issue 5039 - add support of nullable non-object values in settings

include/AdblockPlus/JsEngine.h

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2017 eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 168 matching lines @@
 #endif
     //@}
 
     /**
      * Creates a new JavaScript object.
      * @return New `JsValue` instance.
      */
     JsValuePtr NewObject();
 
     /**
+     * Creates a null JavaScript object.
+     * @return New `JsValue` instance.
+     */
+    JsValuePtr NullValue();
+
+    /**
      * Creates a JavaScript function that invokes a C++ callback.
      * @param callback C++ callback to invoke. The callback receives a
      *        `v8::Arguments` object and can use `FromArguments()` to retrieve
      *        the current `JsEngine`.
      * @return New `JsValue` instance.
      */
     JsValuePtr NewCallback(v8::InvocationCallback callback);
 
     /**
      * Returns a `JsEngine` instance contained in a `v8::Arguments` object.
@@ skipping 112 matching lines @@
     EventMap eventCallbacks;
     std::mutex eventCallbacksMutex;
     std::mutex isConnectionAllowedMutex;
     IsConnectionAllowedCallback isConnectionAllowed;
     TimerTasks timerTasks;
     TimerPtr timer;
   };
 }
 
 #endif