Add observer for BAAnnots from Javascript

fpdfsdk/include/cpdfsdk_annot.h

Index: fpdfsdk/include/cpdfsdk_annot.h
diff --git a/fpdfsdk/include/cpdfsdk_annot.h b/fpdfsdk/include/cpdfsdk_annot.h
index 967226e9db382a96364169d1d4f890d6c983f8dc..a43b8775dbbb3b581f67a35999c4324c7c781959 100644
--- a/fpdfsdk/include/cpdfsdk_annot.h
+++ b/fpdfsdk/include/cpdfsdk_annot.h
@@ -23,9 +23,22 @@ class CPDFSDK_PageView;
 
 class CPDFSDK_Annot {
  public:
+  class Observer {
+   public:
+    explicit Observer(CPDFSDK_Annot** pWatchedPtr);
+    ~Observer();
+    void OnAnnotDestroyed();
+
+   private:
+    CPDFSDK_Annot** m_pWatchedPtr;
+  };
+
   explicit CPDFSDK_Annot(CPDFSDK_PageView* pPageView);
   virtual ~CPDFSDK_Annot();
 
+  void AddObserver(Observer* observer);
+  void RemoveObserver(Observer* observer);
+
 #ifdef PDF_ENABLE_XFA
   virtual FX_BOOL IsXFAField();
   virtual CXFA_FFWidget* GetXFAWidget() const;
@@ -57,6 +70,7 @@ class CPDFSDK_Annot {
   void SetSelected(FX_BOOL bSelected);
 
  protected:
+  std::set<Observer*> m_Observers;
   CPDFSDK_PageView* m_pPageView;
   FX_BOOL m_bSelected;
 };