Issue #4694 - Add mutex protection to JS event handling

include/AdblockPlus/JsEngine.h

Index: include/AdblockPlus/JsEngine.h
===================================================================
--- a/include/AdblockPlus/JsEngine.h
+++ b/include/AdblockPlus/JsEngine.h
@@ -40,6 +40,11 @@
   typedef Handle<Value>(*InvocationCallback)(const Arguments &args);
 }
 
+/*
+ * Forward declaration of internally-defined implementation class
+ */
+class JsEngineInternal;
+
 namespace AdblockPlus
 {
   class JsEngine;
@@ -281,6 +286,12 @@
      */
     JsValuePtr GetGlobalObject();
 
+    /**
+     * Provide a reference to this object in its type as constructed.
+     * Objects of this type are only ever constructed as `JsEngineInternal`.
+     */
+    JsEngineInternal& Internal();
+
     /// Isolate must be disposed only after disposing of all objects which are
     /// using it.
     ScopedV8IsolatePtr isolate;
@@ -288,7 +299,6 @@
     FileSystemPtr fileSystem;
     WebRequestPtr webRequest;
     LogSystemPtr logSystem;
-    EventMap eventCallbacks;
 
     // Forward declaration for PImpl idiom
     class SchedulerImpl;